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