effects: attempt to screenshot on OpenGL ES 2.0 instead of failing
Summary:
Current code path was attempting to use both framebuffer blit and
glReadPixels on OpenGL ES, instead change the code to use framebuffer
blit and glGetTexImage on OpenGL and glReadPixels on the OpenGLES as it
doesn't have glGetTexImage available.
Test Plan: tested on Nexus 5X.
Reviewers: KWin, davidedmundson, graesslin
Reviewed By: KWin, graesslin
Subscribers: kwin
Tags: KWin
Maniphest Tasks: T10011
Differential Revision: https://phabricator.kde.org/D16802