CanvasSource coordinates inverted in 0.40.0

[gpbmike]

mapbox-gl-js version: 0.40.0

Steps to Trigger Behavior

1. Add canvas layer to map with coordinates order top left, top right, bottom right, bottom left per spec.

Expected Behavior

Canvas layer should render in desired location.

Actual Behavior

Canvas layer is rotated and flipped. If you use the following order the canvas layer will render correctly: bottom left, bottom right, top right, top left.

Here's a giphy that compares v0.37.0 (first, smooth fps) to v0.40.0 (second, choppy fps). Notice the colored layer is positioned differently in the second example.

Framerate issues addressed here: #5301

[lbud]

omg the internet 💀 — webgl.readPixels and canvas ImageData are inverted 🤕 so adding an intermediary image buffer to mitigate GPU-specific rendering issues introduced this, and I apparently didn't have any vertically asymmetrical test canvas pages to catch this 🤦‍♀️

Fixing in #5303, which will be included in a patch release within the next week.