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VSE strip thumbnail drawing function

Thumbnail Drawing Function Improvements T89144

The previous version drew the thumbnails but didn't take into consideration
The visible range where the thumbs should be seen and tried to draw till
end of strip. This version considers only those thumbails which can be seen
and leads to large performance gain. Multiple strips can be used smoothly.
Drawing logic was change to keep same thumbs when strip slide operation was
performed.

Differential Revision: https://developer.blender.org/D11616