Inherit NCollection_Buffer from Standard_Transient, do not use incomplete NCollection_Handle.
OpenGl_Context, add methods ActiveProgram(),BindProgram()
to manage currently active GLSL program.
Add method ::ToUseVbo().
OpenGl_ShaderProgram, setup locations of pre-defined vertex attributes
occVertex, occNormal, occTexCoord, occColor before linkage.
Remove methods OpenGl_ShaderProgram::Bind(), ::BindWithVariables()
and ::Unbind() - OpenGl_Context::BindProgram() should be used instead.
Introduce class OpenGl_VertexBufferCompat, which emulates VBO
behavior on systems without VBO (compatibility with broken OpenGL drivers on Windows).
OpenGl_PrimitiveArray - use OpenGl_VertexBufferCompat when VBO is unavailable,
remove duplicated code. Use OpenGl_VertexBuffer::HasNormalAttribute()
method to activate lighting.
OpenGl_Text - use OpenGl_VertexBufferCompat, eliminate duplicated code.
Changes in OpenGl_VertexBuffer, drop methods BindFixed()/UnbindFixed().
Superseded by new methods BindAllAttributes()/UnbindAllAttributes()
which handle active GLSL program, when it is set.
