Explicitly request alpha channel for GL test backbuffer

util/test/demos/gl/gl_test_linux.cpp

@@ -134,6 +134,8 @@ void *OpenGLGraphicsTest::MakeContext(GraphicsWindow *win, void *share)
                              8,
                              GLX_BLUE_SIZE,
                              8,
+                             GLX_ALPHA_SIZE,
+                             8,
                              GLX_DOUBLEBUFFER,
                              True,
                              GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB,

util/test/demos/gl/gl_test_win32.cpp

@@ -89,6 +89,7 @@ bool OpenGLGraphicsTest::Init()
   pfd.cColorBits = 24;
   pfd.cDepthBits = 0;
   pfd.cStencilBits = 0;
+  pfd.cAlphaBits = 8;
 
   int pf = ::ChoosePixelFormat(dc, &pfd);
   ::SetPixelFormat(dc, pf, &pfd);