- move shader flags from the video drivers to the context drivers
- rework config_load_shader_preset() from configuration.c into retroarch_load_shader_preset() in retroarch.c with proper compatibility check
- implicitly call retroarch_load_shader_preset() in retroarch_get_shader_preset() once per core/content life cycle
- use video_shader_is_supported() instead of video_driver_test_all_flags() where appropriate
- remove GFX_CTX_FLAGS_NONE because it is meaningless