Intel OpenGL drivers have an issue where the results of a series of
glBlitFramebuffer() can delay updating the display, when we use GDK_GL=always,
which is manifested when attempting to enter text in text boxes.

This attempts to work around this issue by requiring a glFlush() call and a
retry to the same glBlitFramebuffer() calls to avoid delays in keystrokes when
using GDK_GL=always and when not using libANGLE OpenGL/ES emulation, when an
Intel OpenGL driver is being used.

Special thanks to Lukas K. for the analysis and coming up with a workaround,
which this patch builds upon.

Fixes issue 3487