python/regress: Use X8R8G8B8 rendertarget in vertex shader test.

src/gallium/state_trackers/python/tests/regress/vertex-shader/vertex-shader.py

@@ -112,7 +112,7 @@ def test(dev, name):
 
     # framebuffer
     cbuf = dev.texture_create(
-        PIPE_FORMAT_A8R8G8B8_UNORM, 
+        PIPE_FORMAT_X8R8G8B8_UNORM, 
         width, height,
         tex_usage=PIPE_TEXTURE_USAGE_DISPLAY_TARGET,
     ).get_surface()