video_core: Arithmetic overflow warning fix for gl_rasterizer (#1262)

* video_core: Arithmetic overflow fix for gl_rasterizer

- Fixed warnings, which were indicating incorrect behavior from integral
promotion rules and types larger than those in which arithmetic is
typically performed.

- Added const for variables where possible and meaningful.

* Changed the casts from C to C++ style

Changed the C-style casts to C++ casts as proposed.
Took also care about signed / unsigned behaviour.