Started FXT1 and ETC2 decompression support ...

I copied part of the Mesa3D "texcompress_fxt1.c" file to "pxfmt_fxt1.cpp".  I
copied over the "texcompress_etc.c" (i.e. mostly ETC2) support.  I'm not
planning to support ETC1, because it's for OpenGL-ES 1.x.  I took only the
decompression functions.

I put some common macros, functions, etc. (i.e. that the Mesa3D code uses) into
"pxfmt_internal.h" and "pxfmt_internal.cpp".

I noticed that the LunarG copyrights were split across 2 lines, and that didn't
mesh well with the Mesa3D copyrights.  I updated the copyrights.

It compiles on Linux and Windows.

I tried it on some non-compressed KTX files, plus some DXT-based KTX files.
This all still works!

I need some KTX files with ETC2/FXT1 textures in them, so that I can test.