Allow updating textures in PostProcessStage

[OmarShehata]

Similar to #5080, if you are passing a canvas as a uniform to a post process, it will not be updated even if the canvas changes. I believe this is by design, but there doesn't seem to be a way to explicitly update it with the public API.

This Sandcastle will draw a red/blue rectangle to a canvas, and it will force the PostProcessStage to re-create the texture with this private API hack:

stage._texturesToCreate.push({
            name: 'myTexture',
            source: document.querySelector('#customCanvas')
        });

We could make PostProcessStage handle setting it directly like this:

stage.uniforms.myTexture = document.querySelector('#customCanvas');

And it would behave as it does in the above Sandcastle. Doing this currently will crash because it expects that uniform to be a Cesium.Texture object at that point.

Came up on the forum.

[OmarShehata]

@lilleyse can we tag this category - graphics and type - enhancement ?