Apply draw offset in vertex shaders; data-list reverse button, editbox scrolling

[dhardy]

The clip-region offsets are now applied by the vertex shaders, thus:

• DrawHandle::draw_device no longer returns an Offset which needs to be applied in user and theme code.
• The bg_common bind group contains scale and offset data and must be updated for each clip region. This is a fairly small extra cost, and might instead use push-constants later. This change will be helpful if we decide to add indirect rendering to textures in the future.
• To avoid precision loss (visually apparent as "drunk text"), the maximum offset is reduced from 2^31 to approx 2^24. This is still sufficient for most purposes and should be possible to work around for ListView.

Additionally,

• The gallery now uses a ScrollLabel for the multi-lingual text label, enabling selection
• EditField now updates its required field on text edits, thus fixing scrolling of contents
• data-list and data-list-view examples have reverse-direction buttons (with fixes for reversed ListView)
• added Directional::Reversed type and reversed method