Texture API issues

Some observations of the current texture API:

• setSource only works with 'browser interface' objects (not arrays of data) and the function correctly tracks which mipmap levels are dirty using _levelsUpdated.
• getSource will return browser interface objects and also arrays of data which it strictly probably shouldn't (since it is matched with setSource).
• lock and unlock work with arrays data and the entire texture is always re-uploaded if any mipmap level is locked, whether for read or write.

Ideally we'd have an API that:

• has a single way of updating both types of texture data
• tracks which face/mipmap levels have been updated and only upload those to GPU
• makes it simple to get, set and dirtify individual faces/levels

Also, the current API assumes textures have CPU-side data which is uploaded to GPU. However we often have GPU-side texture data which we want to copy back to CPU.

@mvaligursky are there any additional restrictions or requirements imposed by webgpu on texture handling?