Try to fix problems caused by (auto)save's screenshots
Added code to allow falling back on stbi for when libpng/libjpeg are not compiled it, at eukara's request.
Handle .exr image files as suggested by eukara, when the appropriate library is available.
Fix mipmaps etc for half-float files.
Enable support for stbi's special gif loader, loading gifs as an 2darray texture.
Add code for threading the qcvm's tempstring recycling, disabled due to paranoia but does otherwise help xonotic perf (at the cost of extra ram).
git-svn-id: https://svn.code.sf.net/p/fteqw/code/trunk@5548 fc73d0e0-1445-4013-8a0c-d673dee63da5
cfd20f4f06