The as_any() method returning a `&mut dyn Any` meant that the type has to be
'static. The class solution of returning &()
doesn't work because it has to be a mutable ref. Therefore just return an Option.
Now that we store the layer textures as Rc<Texture>, before rendering a
layer we can take a clone of a possibly existing layer texture and if if
the layer's tracker is dirty, we can re-use the existing layer texture
if it's the same size.
This avoids unnecessary texture allocations.
Clean up texture caching in the GL renderer
In preparation for moving the image decoding and caching of decoded
image data into the core library, change the texture cache to be really
a cache of textures. Previously it cached Rc<CachedImage>, which could
have also been just a CPU side image. However this was "run-time"
asserted. Instead, with this patch it's an `Rc<Texture>` and that will
always be GPU side.
