Allow creation of Textures from existing image data

[davesmith00000]

Apologies if this is already supported or due to be released, I couldn't find any references, but I think it's sort of related to the open PR about creating textures from bytes.

Textures can be loaded from a url, but I'd like to be able to read image data directly from an image I've already loaded, or a 2D canvas that is already on the page.

[shamansir]

Could be the brother issue of this one: #6. Should not the name confuse you, since the Pull Request it was created from, supported exactly what you describe: elm-community/webgl#45.