Add premultiply option for texture loading

In order to alpha blend textures correctly, their color channel needs to be premultiplied by the alpha channel. I believe WebGL supports doing this when loading textures but that feature isn't exposed in this package.

For that reason I think `premultiply : Bool` should be added as an additional field available when calling `WebGL.Texture.loadWith`. For `WebGL.Texture.load` the default value will be `False`.