apps/gl: Fix default state.
apps/gl/default: MacOSX software renderer only supports 8 fixed function textures.
apps/egl/gl: Skip when failed to create window.
apps/gl: Drop the single buffered test.

GLFW does not support single buffered visuals, and they are out of use
anyway.
apps/gl: Add a specific test for default state.
tracemeld: Allow to override diff tool.
apps/egl: Don't expect EGL_KHR_create_context to be always supported.
apps/gl: Attempt to automatically discover app tests (WIP).
apps/gl: More test discovery.
apps/gl: Use add_app_test() macro.
apps/gl: Just rely on //! annotations.