Add ctx support to bl_bench

[hodefoting]

This adds benchmarking support for ctx which is a highly portable 2D rasterizer, relying on C compilers and auto-vectorization to do heavy lifting.

Performance is highly dependent on the compiler (and compiler flags). Ctx seems to work best with clang in general ~5-10% faster than gcc. For one particular case clang does much better FillRectU bi-linear. Where clang yields code 3-4x faster than gcc.

For building and installing ctx, hopefully the following works:

git clone https://ctx.graphics/.git/
cd ctx.graphics
./configure.sh
CC=clang make -j && sudo make install