Use flags for gpu-flags-constant

[MendyBerger]

These should be flags

• gpu-buffer-usage
• gpu-texture-usage
• gpu-shader-stage
• gpu-color-write
• gpu-map-mode

These should be removed:

• gpu-buffer-usage-flags
• gpu-texture-usage-flags
• gpu-shader-stage-flags
• gpu-color-write-flags
• gpu-map-mode-flags

gpu-flags-constant should also be removed.