Cache hillshade textures based on texture size, not tile size.

[ChrisLoer]

Fixes #7690 -- hillshade layers leak GPU memory.

I tested this manually by panning around in the hillshade.html debug page. Before the change, you can see painter._tileTextures fill up with an unbounded set of textures and watch memory use grow. After the change, memory use stabilizes after panning around for a while, and you can watch textures getting re-used in Painter#getTileTexture.

@mollymerp it doesn't look to me like native has the same problem, but not sure if the logic could be hidden somewhere else -- does that sound right to you?

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

[mollymerp]

thank you for tracking this down! yep I double checked and native does not have the same issue.