Destroying a component releases the item's graphics resources, which requires
a mutable reference to the rendering cache. That reference is acquired fine grained,
but when rendering items itself we also acquired a mutable reference
at the start of the rendering phase. That is too early and too long, and not necessary.
Similar to the mouse_grabber, we use a VisitChildrenResult field to
track the focus item within a component. Unlike the mouse grabber
however, it is set/cleared using dedicated focus events.
The key event now routes the key event directly to the focus item.
The focus can be requested via set_focus_item on a window, which the
TextItem does.
In the future the TextInput will request focus on mouse click,for
example.
Pass the outer-most component through to ItemVTable's input_event.
For the purpose of disambiguating this component from any nested
component instantiated by a repeater or so, it's called the
app_component.
The ComponentVTable takes a reference to a ComponentRefPin instead of a
ComponentRefPin by value, as the vtable macro gets confused otherwise
and thinks it's a self argument.
QPainter does not like to draw in an empty QImage.
And the rendering code does not like to render it.
So we would get this on the console:
```
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::save: Painter not active
QPainter::save: Painter not active
QPainter::setRenderHint: Painter must be active to set rendering hints
QPainter::save: Painter not active
QPainter::setClipRegion: Painter not active
QPainter::setPen: Painter not active
QPainter::setBrush: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::setPen: Painter not active
QPainter::setBrush: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::save: Painter not active
QPainter::restore: Unbalanced save/restore
QPainter::restore: Unbalanced save/restore
thread 'main' panicked at 'attempt to subtract with overflow', sixtyfps_runtime/rendering_backends/gl/texture.rs:285:42
```
Make the text color a rendering variable, so that it can be passed
through as uniform to the glyph shader and applied to the gray
map of the glyphs. This avoids re-creating the glyph runs when
merely the color changes.
This already "worked" for the glyph cache based text rendering,
but it wasn't used because of the wasm canvas code path. This
patch changes that to render the text into a text using simply black
and then render that texture using our existing glyph shader,
which merely uses the alpha channel anyway.
This reduces also #cfg's.
Currently the Qt backend still redirect everything to the GL backend,
but the goal is to use QPainter and QWindow
This also adds a "default" backend, whose goal is to select the proper
backend at compile time
Borrow the rendering and cache individually with different mutability.
We might later, for destruction handling, need to borrow the rendering
cache mutably. That in turn *may* happen even during rendering itself,
as the models are updated lazily during visitor traversal and that may
result in item destruction.
Instead, pass a reference to the root item when mapping the window,
at which point we can downcast to the new Window item. If we have one,
then we'll read its width/height (for initial values) and install
bindings to keep them up-to-date.
