Replaced ctx->Color._DriverDrawBuffer with swrast->CurrentBuffer.
Replaced ctx->Pixel._DriverReadBuffer with ctx->Pixel._ReadSrcMask.
swrast->Driver.SetBuffer() takes FRONT/BACK_LEFT/RIGHT_BIT values now.
Added tokens and code for GL_AUX buffers, for completeness.
Also changed parameter types for some driver functions (like ctx->Driver.Clear-
Color). Updated all the device drivers.
Someday, we want to support 8, 16 and 32-bit channels dynamically at runtime.