Support for addTexture or some kind of dynamic/mutable texture support on web

[eyebrowsoffire]

As discussed in flutter/engine#37890

Now that we have the createImageFromImageBitmap API in dart:ui_web, users can create images from arbitrary browser image sources. However, creating a completely new image (and therefor WebGL texture) every time can be inefficient in some cases, and it would be preferable to be able to update the contents of an existing texture. We could expose this either through the addTexture API or through some other mechanism.

[jezell]

@eyebrowsoffire this seems to do the trick without needing a new API:

flutter/packages#6286

Though I think the API is a little off for createImageFromImageBitmap as it has bad type checks and also is missing a bunch of params that the browser API supports relating to sizing and cropping of the texture. Had to hack around it a little.

#144815