[Impeller] fix partial repaint texture sizing.

[jonahwilliams]

Locally this fixes the embedder support regression caused by partial repaint textures being too large.

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[zanderso]

A bit confused. Wasn't this the fix for the previous regression? 3aebe09.

[jonahwilliams]

that's wrong then ...

We should revert for now until we figure this out I guess

[jonahwilliams]

There may be some confusion as to whether we're wrapping the surface texture or creating a new texture. Whenever we perform a partial repaint, the surface texture isn't used and instead a new texture is created that is a smaller size.

[jonahwilliams]

closing this for now, but the lack of this functionality is the cause of the current regression

[bdero]

My attempted fix was indeed wrong, but there's another bug in here where I'm re-wrapping the texture with an incorrect descriptor.