Check GLEW functions exist before calling them

Fixes https://gitlab.com/kicad/code/kicad/-/issues/6235

common/gal/opengl/cached_container_gpu.cpp

@@ -67,6 +67,7 @@ CACHED_CONTAINER_GPU::~CACHED_CONTAINER_GPU()
     if( m_isMapped )
         Unmap();
 
+    if( glDeleteBuffers )
         glDeleteBuffers( 1, &m_glBufferHandle );
 }
 
@@ -75,6 +76,10 @@ void CACHED_CONTAINER_GPU::Map()
 {
     wxCHECK( !IsMapped(), /*void*/ );
 
+    // OpenGL version might suddenly stop being available in Windows when an RDP session is started
+    if( !glBindBuffer )
+        throw std::runtime_error( "OpenGL no longer available!" );
+
     glBindBuffer( GL_ARRAY_BUFFER, m_glBufferHandle );
     m_vertices = static_cast<VERTEX*>( glMapBuffer( GL_ARRAY_BUFFER, GL_READ_WRITE ) );
 

common/gal/opengl/cached_container_ram.cpp

@@ -51,7 +51,9 @@ CACHED_CONTAINER_RAM::CACHED_CONTAINER_RAM( unsigned int aSize ) :
 
 CACHED_CONTAINER_RAM::~CACHED_CONTAINER_RAM()
 {
+    if( glDeleteBuffers )
         glDeleteBuffers( 1, &m_verticesBuffer );
+
     free( m_vertices );
 }
 

common/gal/opengl/gpu_manager.cpp

@@ -85,7 +85,10 @@ GPU_CACHED_MANAGER::~GPU_CACHED_MANAGER()
 {
     if( m_buffersInitialized )
     {
+        if( glBindBuffer )
             glBindBuffer( GL_ARRAY_BUFFER, 0 );
+
+        if( glDeleteBuffers )
             glDeleteBuffers( 1, &m_indicesBuffer );
     }
 }

common/gal/opengl/opengl_compositor.cpp

@@ -400,7 +400,10 @@ void OPENGL_COMPOSITOR::clean()
 
     m_buffers.clear();
 
+    if( glDeleteFramebuffersEXT )
         glDeleteFramebuffersEXT( 1, &m_mainFbo );
+
+    if( glDeleteRenderbuffersEXT )
         glDeleteRenderbuffersEXT( 1, &m_depthBuffer );
 
     m_initialized = false;

common/gal/opengl/shader.cpp

@@ -58,10 +58,12 @@ SHADER::~SHADER()
         Deactivate();
 
     if( isProgramCreated )
+    {
+        if( glIsShader )
         {
             // Delete the shaders and the program
-        for( std::deque<GLuint>::iterator it = shaderNumbers.begin();
+            for( std::deque<GLuint>::iterator it = shaderNumbers.begin(); it != shaderNumbers.end();
-                it != shaderNumbers.end(); ++it )
+                    ++it )
             {
                 GLuint shader = *it;
 
@@ -75,6 +77,7 @@ SHADER::~SHADER()
             glDeleteProgram( programNumber );
         }
     }
+}
 
 bool SHADER::LoadShaderFromFile( SHADER_TYPE aShaderType, const std::string& aShaderSourceName )
 {