Reload Shm texture when buffer size changes
Summary: Shm textures via Gl must be reloaded in case their size changes.
Test Plan: testShmBufferSizeChangeOnSubSurface passes.
Reviewers: KWin, graesslin
Reviewed By: KWin, graesslin
Subscribers: davidedmundson, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D18453