Cleaned up warning messages in GC3D
Reorganized texture loading. Fixed support for I8_A8 textures.

- Now we only load texture data once, instead of once for each reference in a Texture Header
- Fixed I8_A8 support by plumbing a bitfield through to the shader. Each bit in the bitfield (GisAflags) is set based on the texture format of the texture bound to that slot. If it's set, then we should treat the G channel of the sample as the A channel. This is needed because DXGI doesn't have a native R8A8 format.