[WIP] Image masked screen rectangle

[kvark]

Includes #529
Renames local_rect -> screen_rect for the ImageMaskData, since the idea now is that the mask is always screen-aligned, and we are baking anything non-aligned into the off-screen textures (#498).

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This change is

[kvark]

Discussed with @glennw . It appears that we first need the image masking shader logic to work properly in local space. That's going to be a small change. And then, we'll be introducing the masking tiles (in screen space) to either replace the local image masking or be optional alternative to it.