* ENH: separate png from opengl, by splitting it into stb and opengl_image modules
Added a unit test for PNG,BMP,TGA,JPG reading and writing
Renamed module png to stb
Renamed functions readPNG to read_image and writePNG to write_image
* name folder by dependency; consistent arg order
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Co-authored-by: Alec Jacobson <alecjacobson@gmail.com>