Blender Git Loki

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

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