mesa: Use appropriate call convention in VertexAttribI*v.

mesa: Use appropriate call convention in VertexAttribI*v.

We were using typecasts because the functions pointers are polymorphic
in the second argument type, which.

Surprisingly the wrong calling convention didn't cause crashes on Windows,
but it was causing certain registers to be trashed in MSVC optimized
builds, when processing callists in the ClearView RC Flight Simulator.