Introduce the DirtyRect type #3277

Before this PR each tile stores its own dirty rect relative to the entire image. This is somewhat confusing because this field is used to track partial uploads correctly and uploads reason about dirty rects relative to the tiles so it has to do some conversions. Storing relative to the tile is also truer to the "device space" the rects are tagged with. Last but not least the image-relative coordinates will make even less sense for tiled blob images as we start trating them as virtually infinite planes.

Previously dirty rects were set up in a subtle way where early in the pipeline None is used to mean "no information" and therefore "all clean", but then later on is used to mean "all dirty" on the assumption that if a texture is still being handled, some dirty information must have been sent. None is therefore used by the internals as a way to signal things like missing cache entries without actually bothering to look up the dimensions of the image (note that "clean" can always be expressed without knowing dimensions, as it is always the same empty rect). I change the system to just consistently use a custom enum that has AllDirty and SomeDirty(empty_rect) to represent these two states. As a result the code seems much easier to understand.

Blobs will need some extra APIs that don't apply to regular images. This change will make it easier to add these extra functionalities without messing with the rest of the code. The new BlobImageKey type also adds a tiny bit of type safety to prevent some mixing blob and non blob data on the same image which is invalid.

…ce cache.

servo / webrender

Introduce the DirtyRect type #3277

Introduce the DirtyRect type #3277

Commits on Nov 23, 2018