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/OSD/OSD_Chronometer.cxx

@@ -51,7 +51,7 @@
 void OSD_Chronometer::GetProcessCPU (Standard_Real& theUserSeconds,
                                      Standard_Real& theSystemSeconds)
 {
-#if defined(__linux__) || defined(__FreeBSD__) || defined(__ANDROID__) || defined(__QNX__)
+#if defined(__linux__) || defined(__FreeBSD__) || defined(__ANDROID__) || defined(__QNX__) || defined(__EMSCRIPTEN__)
   static const long aCLK_TCK = sysconf(_SC_CLK_TCK);
 #else
   static const long aCLK_TCK = CLK_TCK;

src/OSD/OSD_MemInfo.cxx

@@ -116,7 +116,15 @@ void OSD_MemInfo::Update()
       myCounters[MemHeapUsage] += hinfo._size;
   }
 
-#elif (defined(__linux__) || defined(__linux))
+#elif (defined(__linux__) || defined(__linux) || defined(__EMSCRIPTEN__))
+  const struct mallinfo aMI = mallinfo();
+  myCounters[MemHeapUsage] = aMI.uordblks;
+#if defined(__EMSCRIPTEN__)
+  // /proc/%d/status is not emulated - get more info from mallinfo()
+  myCounters[MemWorkingSet]     = aMI.uordblks;
+  myCounters[MemWorkingSetPeak] = aMI.usmblks;
+#endif
+
   // use procfs on Linux
   char aBuff[4096];
   snprintf (aBuff, sizeof(aBuff), "/proc/%d/status", getpid());
@@ -162,10 +170,6 @@ void OSD_MemInfo::Update()
     }
   }
   aFile.close();
-
-  struct mallinfo aMI = mallinfo();
-  myCounters[MemHeapUsage] = aMI.uordblks;
-
 #elif (defined(__APPLE__))
   struct task_basic_info aTaskInfo;
   mach_msg_type_number_t aTaskInfoCount = TASK_BASIC_INFO_COUNT;

src/OSD/OSD_Path.cxx

@@ -39,6 +39,8 @@ static OSD_SysType whereAmI()
   return OSD_VMS;
 #elif defined(__linux__) || defined(__linux)
   return OSD_LinuxREDHAT;
+#elif defined(__EMSCRIPTEN__)
+  return OSD_LinuxREDHAT;
 #elif defined(_AIX) || defined(AIX)
   return OSD_Aix;
 #else

src/OSD/OSD_signal.cxx

@@ -703,7 +703,7 @@ typedef void (* SIG_PFV) (int);
 
 #include <signal.h>
 
-#if !defined(__ANDROID__) && !defined(__QNX__)
+#if !defined(__ANDROID__) && !defined(__QNX__) && !defined(__EMSCRIPTEN__)
   #include <sys/signal.h>
 #endif