Add premultiply option for texture loading

[MartinSStewart]

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.