Do not write alpha to the main framebuffer when performing antialiasing.

This allows correct background display when the main framebuffer has alpha.
Noticeable on Wayland with Mesa 24.0.2 Iris driver.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17341

common/gal/opengl/antialiasing.cpp

@@ -185,7 +185,11 @@ void ANTIALIASING_SUPERSAMPLING::Present()
     glBindTexture( GL_TEXTURE_2D, compositor->GetBufferTexture( ssaaMainBuffer ) );
     compositor->SetBuffer( OPENGL_COMPOSITOR::DIRECT_RENDERING );
 
+    glColorMask( GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE );
+
     draw_fullscreen_primitive();
+
+    glColorMask( GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE );
 }
 
 
@@ -547,7 +551,11 @@ void ANTIALIASING_SMAA::Present()
     glActiveTexture( GL_TEXTURE1 );
     glBindTexture( GL_TEXTURE_2D, blendTex );
 
+    glColorMask( GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE );
+
     pass_3_shader->Use();
     draw_fullscreen_triangle();
     pass_3_shader->Deactivate();
+
+    glColorMask( GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE );
 }

common/gal/opengl/opengl_compositor.cpp

@@ -297,7 +297,7 @@ void OPENGL_COMPOSITOR::ClearBuffer( const COLOR4D& aColor )
 {
     wxASSERT( m_initialized );
 
-    glClearColor( aColor.r, aColor.g, aColor.b, 0.0f );
+    glClearColor( aColor.r, aColor.g, aColor.b, m_curFbo == DIRECT_RENDERING ? 1.0f : 0.0f );
     glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT );
 }