Conflicts:
	stb_image_write.h

to avoid probelm with clang -O2 to outputting buggy code

Fixes #404

My clang doesn't like the macro defined this way, choking at the callsite on line 195 with "too many arguments provided to function-like macro invocation"

This change matches what is done for STBTT_memset in stb_truetype.h

Someone should double check that that I didn't change
the behavior of any of the code.  I'm not using most (if any)
of the code I touched, just wanted it to compile...

(GCC recognizes certain strings in comments)

* `force_filter` being < 0 means the original behavior (i.e. figure out
  the best-performing filter per scanline); any other values 0 <= x <= 4 correspond
  to PNG filters (0 = none, 1 = sub, 2 = up, 3 = average, 4 = Paeth).
* `compression_level` being < 0 equals `compression_level` 8 (the previous value).
  The higher this is, the better the compression should be (though it will use
  more memory).

These new parameters are not (yet) exposed for the higher-level API functions.

This prevents a "Conditional jump or move depends on uninitialised
value(s)" error from valgrind when using zero as an argument in line
1045.