Auto merge of #1529 - nical:update-ext-img, r=kvark
Auto merge of #1529 - nical:update-ext-img, r=kvark

Implement updating external image buffers.

Initial implementation that ignores the dirty rect, which is correct but just misses the potential optimization of uploading a sub-rectangle of the image. Right now gecko is not wired up to send the dirty rect so I don't have a good way to test the optimization handy and I want to make progress on image updates as soon as possible.

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