Retexturing support is now always on (you can still switch it off with
the gl_retexturing CVAR), as we don't have the additional libjpeg
dependency anymore.
stb_image is used for tga, jpg and the newly supported png, so the
old tga and jpeg loading code has been removed.
I furthermore cleaned up the somehow messy and possibly slightly broken
retexturing selection code in R_FindImage()
Without this define newer versions of jpeg define a type "bool" as char,
while the MinGW headers define it as "unsigned short". Automake should
have detected that, but...
