Support for WebGLTexture target

[mauricef]

Thank you for creating and sharing this! I would like to pass in a WebGLTexture as a uniform but that does not appear to be supported. The API documentation says that the target parameter to glsl can be a WebGLTexture but that didn't seem to work either and looking through the swissgl code, I don't see a path where it will construct a TextureTarget from a WebGLTexture.