Merge pull request #1 from nothings/resample

stb_resample updates
Merge pull request #3 from nothings/resample

Resample
Fix malloc context and whitespace.
Progress report.
Clarify some comments. Make stbir__resize_arbitrary an internal function. Update test cases to use actual API functions.
Correctly specify alpha channels.
Merge branch 'resample2' into resample