BGFX universal renderer

[rb6502]

Phase 1, replace current GLSL/HLSL with reasonable cross-platform common denominator.

• Must be data-driven and support multiple passes with multiple shaders
• Support both a built-in pipeline analagous to current HLSL system and a user-supplied pipeline with user-defined shaders

Phase 2, polygonal rasterization support:

1. Ability to render polygonal game screen to an off-screen buffer, then upscale the final image
2. Need ability to pass down triangle strips and fans from the driver
3. Driver should be able to supply a custom pixel shader to get arcade-correct results for more unusual h/w
4. Driver should be able to do some texture management; e.g. Namco S22 on modern video cards will want to upload the entire texture sheet once at the beginning for max performance

This will be involved with the "3D Artwork System" item as well.