Prefer pkg-config for lib flags where possible
Port over Bitmap 'radial_blur' from old SFML codebase

This implementation is also heaps better than the old
one as it doesn't use a (differently sized) aux texture,
meaning the Bitmap discards its old texture and aquires
one of same size, making reuse through the TexPool a
lot more likely. It also saves on the aux texture blits
and binding switches.

As the setup / resource acquisition far outweighs the
actual rendering cost, operation time is relatively
constant no matter how many divisions are used.
Fix minor GPU timer query issue
Merge branch 'bitmap_blur'