Pass drive type to allocate_letter().  Start search from 'A' for
floppies.
Fix missing parentheses in the DRIVE_MASK_BIT definition.  It's a bug
that is triggered by non-sequential drive allocation.

I missed a register mask in the move to share the shader_hw_def()
function between pixel and vertex shaders for ARB shaders.  Fixed
that, and made the GLSL version use the same mask for consistency.

so remove the case for it being NULL.

tell the RPC runtime that the call failed.

CoRevertToSelf on top of CoGetCallContext, which is a stub.