Makefile fixes

Link libgomp statically, MinGW library order
Fixed invalid gamma when input/output gamma differs
Version bump
Moved hash state to acolorhash_table
Moved edge/noise bitmaps to image struct
Image mem cleanup
Modified image flag
Moved libpng critical section down
Exposed hash alloc/free methods
acolorhash_table holds maxcolors and ignorebits
Explicit palette generation/remapping steps
Output image mem cleanup
Moved hist hash table out of pam
Moved palette sorting to quantization step
Changed internal gamma to 0.55
LUT for gamma
Moved creation of row_pointers to rwpng
Fixed handling of row pointers when saving 24-bit image
Added sRGB chunk
Default filter on 24-bit images