ctx,uctx: add image drawing support

This drags in stb_image.h adds only the jpg and png loaders, and implements the first part of an ctx.image(path,x,y,width,height) call where call arguments are optional.

Still todo: Further arguments can be added for clip_x, clip_y, clip_width and clip_height - for using a sub-rectangle of the image as the source texture for use with spritesheets.