windowthumbnail: Use mipmap texture filtering

Authored by fredrik on Jan 28 2018, 3:05 PM.

Description

windowthumbnail: Use mipmap texture filtering

Blit the contents of the TFP texture to a separate mipmap texture,
and (re)generate the mipmaps on each damage event.

This commit also fixes ARGB thumbnails being rendered as opaque.

Note that this commit only modifies the GLX path.

BUG: 390457