[Impeller] Need a Skia API for loading emoji fonts directly and bypassing SkCanvas

[jonahwilliams]

In order to render emoji, we need to populate a glyph atlas using an SkCanvas. The software rasterization of these images (frequently pngs) is quite slow and may not be necessary, depending on how the OS APis works. If we are able to load an emoji at the correct scale, it would be advantageous to bypass the software canvas, load the png into a host buffer and blit onto our atlas.

If we need to resize the image, then there may be faster routes than software but its probably less clear.

[jonahwilliams]

If we could just get the png we could bypass all of this...

[jonahwilliams]

#138798

[jonahwilliams]

Basically we need to be able to call something like SkScalerContext_FreeType::generateImage, but give it our own buffer.

[jonahwilliams]

Here is an example of what Skia does on the same app: notice the second round of software rasterization can be avoided:

[jonahwilliams]

Turns out this was our bug. For reasons I don't fully understand, using the Vulkan staging buffer as the canvas src is absolutely destroying the throughput here. We don't actually need this.

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