restorable: Split pixel data into multiple slices

[hajimehoshi]

For sparse images, this should be effective in terms of memory usage. Big shareable images tend to be sparse in our experience.

[hajimehoshi]

I think we can reduce redundant memory usages by these actions:

• Split the byte slice for a texture into a multiple slices (this issue). For example, a 4096x4096 image needed to hold 64MB memory, but if we can keep only needed data, this can be reduced to about 10MB or so.
• Remove CopyPixels command and use ReplacePixels instead. CopyPixels allocates all the pixel data of the texture, the same data might be in the restorable package though. For example, when extending a 2048x2048 image to a 4096x4096 image, CopyPixels read 16MB data from GPU to copy the pixels.

Then, I estimate we can reduce 40-50MB peak memory usage in theory in usual cases.

[hajimehoshi]

Experimental branch: https://github.com/hajimehoshi/ebiten/tree/issue-897-rects