Upgrade miniz and force inline/static use

Upgrades miniz to 1.14 (which incorporates some of our local changes ie MinGW fixes, 64-bit CRC32, etc).

Moves the entire thing into the single-header style (like it was originally) and [forces functions to be static](https://code.google.com/p/miniz/issues/detail?id=18).

The reason for this is that SDL2_image uses miniz internally and exports its symbols, which have the same names as our own local symbols. When dynamic linking this isn't a problem as the runtime linker only looks for missing symbols. When static linking (as we do with the MXE/MinGW builds) this leads to symbol conflicts.

The correct fix is for [SDL2_image not to export these symbols at all](https://bugzilla.libsdl.org/show_bug.cgi?id=2120), but until that happens we need to work around it.