Script: Added resize_image and overlay function

[normaldream]

This allow exporter script (that don't always have control over the size of input images) to do more image operations on input images

[normaldream]

Suggestion:
overlay could be removed if masked_blend (and linear_blend and combine_blend while we're at it) could accept a smaller image as image2 and an (optional) offset, and if there was a get_alpha function