Remove majority of warnings

There were a *lot* of warnings, managed to remove most of them.

Also, put warning flags before C_FLAGS and CXX_FLAGS, rather than after,
as -Wall -Wextra was overwriting flags that came before it.
Fix a some more linux/GCC specific warnings
Fix inttypes.h usage

...I neglected to put a '%' character before using the PRI* macros.
Fix bug with linux build

...Screw it.  %llu it is.
Optimize GL staging surface and fix mac issue

Staging surfaces with GL originally copied to a texture and then
downloaded that copied texture, but I realized that there was really no
real need to do that.  Now instead they'll copy directly from the
texture that's given to them rather than copying to a buffer first.

Secondly, hopefully fix the mac issue where the only way to perform an
asynchronous texture download is via FBOs and glReadPixels.  It's a
really dumb issue with macs and the amount of "gotchas" and non-standard
internal GL functionaly on mac is really annoying.
Fix variable placement, not safe with goto usage