Similar to commit 957186acb7 we need to take the blur into account in the box shadow image:
We add blur margins around the entire image, but we also need to start drawing at (blur, blur) instead of (0, 0).
The same offset needs to be subtracted again when drawing the cached image.
When using box shadows in repeaters, we end up creating multiple
distinct textures of the drop shadow. That's a waste of precious texture
memory if they have the same properties.
Instead, when creating box shadows in a frame, see if they can be
re-used across images. The texture still persistent in the image_cache,
via the explicitly shared skia_safe::Image and Rc<Texture>.
This works well when rendering repeated elements new top-down, but it's
not perfect. For example if a bunch of repeated elements are in a
flickable, only a portion of them are visible and the view is scroll up,
then the top-down rendering will not find a cache hit for the newly
visible element with drop shadows. Yet this is simple enough to help
with sharing in many cases.
Previously: Window is an Rc<WindowInner>, which has an Rc<dyn
PLatformWindow> - and weak references the other way around.
Now: Rc<dyn PlatformWindow> is the root of window ownership. The impl
PlatformWindow has a slint::api::Window, which just holds a WindowInner.
This change is incomplete on a few levels, mainly that neither of the
code generators nor the interpreter is ported.
Because re-generating the Skia image is slow and there is no point
storing both the image buffer and the SkiaImage in the cache as it
is basically the same information.
Colorization is typically implemented using an intermediate layer and since
usually it doesn't change so often, it's faster to cache the result in the existing SkImage cache
per Image item.
It may happen that the clip region becomes empty (for literally invisible items).
For example this happens in the printer demo.
In that case we want to return an empty rect.
