premultiply canvas images when updating texture

[ansis]

Fix #7302 by premultiplying canvas images when updating their textures.

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

[ansis]

I added some render tests that fake canvases using images so that I could add a regression test for this. The approach taken to fake the canvas isn't great but I couldn't think of a better approach.

[ryanhamley]

Awesome! Looks good to me. Can you just explain to me real quick why the texture.bind and gl.texSubImage2D calls aren't necessary anymore?

[ansis]

Can you just explain to me real quick why the texture.bind and gl.texSubImage2D calls aren't necessary anymore?

The call to texture.update(...) contains a gl.texSubImage2D. The other bind calls... I'm not sure that we needed them before. I'm guessing the might be left over from previous refactoring