to validate the conformance values being read from the wire.

exception if a multiply overflows a 4-byte integer.

This will protect the unmarshaling code against attacks specifying a 
large variance.
Use this new function in the conformant string functions to harden them 
against attack.

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.