0031070: Configuration - fix building issues when using Emscripten toolchain

Handled __EMSCRIPTEN__ macros to:
- Workaround atomics (__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 is undefined, but GCC atomics are provided).
- Suppress non-standard header <sys/signal.h> warning.
- Return OSD_LinuxREDHAT.
- Avoid inclusion of XLib headers.
- Skip fontconfig library.
- Enable EGL+GLES path (translated by Emscripten into WebGL).
- Skip eglCreatePbufferSurface() not implemented by Emscripten EGL.

Fixed Graphic3d_Vec4.hxx usage within Quantity_ColorRGBA.hxx.

OpenGl_ShaderManager::defaultGlslVersion() now prefers GLSL 300 es when WebGL 2.0 is available,
as there no any OpenGL ES greater than 3.0 emulation so far.

Shaders_Declarations.glsl - added workaround for GLSL compilation on WebGL 1.0
by defining Light properties accessors as macros instead of functions
('[]' : Index expression must be constant).

OpenGl_FrameBuffer::Init() - added workaround for initialization of GL_DEPTH24_STENCIL8
depth-stencil attachment on WebGL 1.0 + GL_WEBGL_depth_texture extension.

OpenGl_Context::Vec4FromQuantityColor() now considers myIsSRgbActive flag
to handle use case, when Immediate Layer is drawn directly into window buffer,
which is not sRGB-ready.

Added new sample - OCCT WebGL viewer.

src/Aspect/Aspect_DisplayConnection.cxx

@@ -25,7 +25,7 @@ IMPLEMENT_STANDARD_RTTIEXT(Aspect_DisplayConnection,Standard_Transient)
 // =======================================================================
 Aspect_DisplayConnection::Aspect_DisplayConnection()
 {
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
   myDisplay = NULL;
   myIsOwnDisplay = false;
   OSD_Environment anEnv ("DISPLAY");
@@ -40,7 +40,7 @@ Aspect_DisplayConnection::Aspect_DisplayConnection()
 // =======================================================================
 Aspect_DisplayConnection::~Aspect_DisplayConnection()
 {
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
   if (myDisplay != NULL
    && myIsOwnDisplay)
   {
@@ -49,7 +49,7 @@ Aspect_DisplayConnection::~Aspect_DisplayConnection()
 #endif
 }
 
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
 // =======================================================================
 // function : Aspect_DisplayConnection
 // purpose  :

src/Aspect/Aspect_DisplayConnection.hxx

@@ -20,7 +20,7 @@
 #include <TCollection_AsciiString.hxx>
 #include <NCollection_DataMap.hxx>
 
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
   #include <InterfaceGraphic.hxx>
 #endif
 
@@ -39,7 +39,7 @@ public:
   //! Destructor. Close opened connection.
   Standard_EXPORT ~Aspect_DisplayConnection();
 
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
   //! Constructor. Creates connection with display specified in theDisplayName.
   //! Display name should be in format "hostname:number" or "hostname:number.screen_number", where:
   //! hostname      - Specifies the name of the host machine on which the display is physically attached.

src/Aspect/Aspect_FBConfig.hxx

@@ -14,7 +14,7 @@
 #ifndef _Aspect_FBConfig_HeaderFile
 #define _Aspect_FBConfig_HeaderFile
 
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
   typedef struct __GLXFBConfigRec* GLXFBConfig;
   typedef GLXFBConfig Aspect_FBConfig; // GLXFBConfig* under UNIX
 #else

src/Aspect/Aspect_XWD.hxx

@@ -14,7 +14,7 @@
 #ifndef __Aspect_WNTXWD_HXX
 # define __Aspect_WNTXWD_HXX
 
-#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(_WIN32) && (!defined(__APPLE__) || defined(MACOSX_USE_GLX)) && !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
 #  include <X11/XWDFile.h>
 # else