Raise GFXR compiler-warning level

[fabian-lunarg]

we treat warnings as errors, but could/should be more strict.

• e.g. we currently tolerate -Wshadow which can and will eventually manifest as real bugs.
• a good example of where we want to go (at least for gcc) was given by this PR

this issue should track our progress for the transition and group related PRs.

the proposed strategy is as follows:

• we defined a cmake-option USE_STRICT_COMPILER_WARNINGS which will enable stricter warnings
• this option is meant to be only temporary, for the duration of the transition. the default is currently OFF
• in multiple PRs we iterate on both the defined compiler-settings and gradually apply them to our framework
• once applied to all code, we remove the option again and will use the strict settings as new default