Don't use the item's rendering cache to determine the image size, as
that's soley for rendering. The Qt backend doesn't use the item cache
and the GL backend neither after this change. Instead both backends have
a cache for decoded images.
This allows creating multiple windows for example, and it will allow for
showing windows in those tests that require a mapped window.
As a bonus, the run() function on generated components is not consuming
anymore.
The GL backend uses the item graphics cache for the image size function,
which uses a PropertyTracker. That tracker must have the Image's source
included in its dependencies, to avoid that when loading a HTML image
for example, the cache isn't invalidated when the source is changed
before the HTML image was loaded async. That's why the get() call on the
source property must happen from within the PropertyTracker's callback.
For all other items the default is the empty size.
This also required three fixes for the HTML image loading:
* The upload_pending property value was inverted, it needs to start
out as true (pending yes) and be set to false when it finished
loading (not pending anymore)
* Mark the upload_pending property as dirty before scheduling the
redraw, in case it's sync
* Pass the item rendering cache to the image_size function to ensure
that the Rc<CachedImage> is not only weak inside the image_cache
of the GLRendererData but also strong on the item.
Represent a non-specified pixel size and font weight through a None option.
This is cleaner and has the added benefit that the backend can now
easily decide what the default size/weight should be.
For the GL backend a resolved font is just a handle (femtovg::FontId)
and we can cache that. Consequently we don't really need Rc<dyn Font>
but can let `fn font(...)` on the backend return a "resolved" font that
includes the pixel size, which in turn simplifies the Font trait
signatures.
The only caveat is that because on the item level we don't know what the
chosen backend type is, we can only receive a `dyn Font`, which means
it's wrapped - for now - in a Box.
