FindSDL_gfx: Add module to find another SDL component

So far, CMake features Find*.cmake files for several SDL-based libraries such as SDL_mixer or SDL_image, but I was surprised to see no equivalent for SDL_gfx.

A copy of the file created by this PR, FindSDL_gfx.cmake, is already being successfully used in one of my projects, rts.

Fixes: #12004 (closed)