Fix "guaranteed" images not loading.
Add `SEQ_FLAG_SKIP_THUMBNAILS` flag to signify that strip did not
produce thumbnail and so should be skipped when drawing.

Minor cleanup and tweaks

Fix flickering when navigation ends

Set `V2D_IS_NAVIGATING` flag when zooming with scrollbars

Fix alpha on overlap for float images

Finish reusable frame implementation

WIP - Render set of thumbnails that are "guaranteed" to exist.

They are not really guaranteed though, just very likely to exist.

Also math is incorrect when handle is extended beyond content, which is not too nice

wip - reuse thumbnails when scaling

Include changes from P2373

VSE strip thumbnails

The goal is to provide thumbnails in the rectangle coloured strips. Works for
movie clips and image sequences. The thumbnails are loaded from source
using separate thread and stores them in cache. The drawing is called inside the
drawing for each strip, and takes images from cache. Drawing is below the
handles from one end of strip to other, inside view only. The job for caching
is called when images are not available, or there is view change.

All strip operations are valid, and overlap of strip adds transparency to the
image. Images are shown only when strip size is wide enough for clear
visibility of images, and the thumbnails represent source footage clearly.

Cache is limited to 5000 thumbnails and performs cleanup of non visible images
when limit crossed.

Related Task : T89143

Maniphest Tasks: T89143

Differential Revision: https://developer.blender.org/D12266