raster broken by OffscreenCanvas on chrome on MacMini

[astojilj]

After 85517f0, debug/satellite.html shows black tiles on macos 10.15.1 on Chrome. No error in console.

Works fine on Firefox and Safari on the same hardware. Render tests pass.

Mac Mini 2018 3.2 GHz 6-Core Intel Core i7 (Intel UHD Graphics 630 1536 MB)
Chrome Version 78.0.3904.108 (Official Build) (64-bit)

Expected (works fine when offscreenCanvasSupported() returns false):

Actual:

cc @arindam1993 @kkaefer

[astojilj]

Edit:
There is no issue noticeable in hillshade.html. RGBA images are all black in debug/image.html, debug/multiple.html...

Works fine when offscreenCanvasSupported() returns false.

[arindam1993]

Can confirm this is happening on any browser that supports OffscreenCanvas+ImageBitmap, was a result of a major derp wherein the ImageBitmap type check wasn't added in Texture.
This worked fine for everything else (icons, glyphs, hillshade) because we transform those textures into our internal RGBAImage or AlphaImage types.

Fix is out in #9103