GLX: Removed alpha from backbuffer attributes. Fix for NV issue#23.

src/glcontext_glx.cpp

@@ -47,10 +47,10 @@ namespace bgfx
 			GLX_RED_SIZE, 8,
 			GLX_BLUE_SIZE, 8,
 			GLX_GREEN_SIZE, 8,
-			GLX_ALPHA_SIZE, 8,
+//			GLX_ALPHA_SIZE, 8,
 			GLX_DEPTH_SIZE, 24,
 			GLX_STENCIL_SIZE, 8,
-			None,
+			0,
 		};
 
 		// Find suitable config