test-utils: Use a power-of-two size for the FBO

When testing with COGL_DEBUG=disable-npot-textures all of the tests
would fail because the testing infrastructure itself ends up creating
a sliced texture and then trying to use it as a render target. This
just modifies test-utils to use 512x512 for the size of the texture.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

tests/conform/test-utils.c

@@ -3,8 +3,8 @@
 
 #include "test-utils.h"
 
-#define FB_WIDTH 640
-#define FB_HEIGHT 480
+#define FB_WIDTH 512
+#define FB_HEIGHT 512
 
 void
 test_utils_init (TestUtilsGTestFixture *fixture,