Support for offscreen canvas

Hi. I saw you added use of ImageBitmap instead of Image class in a worker, but there is an issue. According to the WebGl spec

UNPACK_FLIP_Y_WEBGL, UNPACK_PREMULTIPLY_ALPHA_WEBGL , UNPACK_COLORSPACE_CONVERSION_WEBGL …
If the TexImageSource is an ImageBitmap, then these three parameters will be ignored. Instead the equivalent ImageBitmapOptions should be used to create an ImageBitmap with the desired format.

invertY has default true in Texture constructor and it cause that some ImageBitmap a flipped.
https://www.babylonjs-playground.com/#08GVKY#8 (to restore Image class its need to reload page)