Scrollables / Clippables

[hecrj]

Content that can take a (practically) infinite amount of space and can be viewed using a scrollbar. This is a very basic feature.

The end-user API could look like this:

Column::new()
    .push(content)
    .scroll(&mut self.scroll_state);

The challenge here is composability. Nested scrollables should work as expected.

When it comes to iced_native, we may need to add the concept of event propagation, so mouse wheel events can be captured by the inner-most scrollable and stop propagation.

When it comes to iced_wgpu, we should be able to use scissoring and transformations. wgpu has a RenderPass::set_scissor_rect that should work nicely.

The main issue here will be wgpu_glyph, which is used currently to render text primitives. wgpu_glyph is powered by glyph-brush, which caches glyphs to render text very fast. However, the current cache model does not seem to be composable (i.e. once you issue a draw call the cache is purged). We will need to change it (and potentially contribute) to accommodate for this use case.