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