This backports the high quality glow mode from the `master` branch.
Previously, during downsample, every second row was ignored.
Now, when high-quality is used, we sample two rows at once to ensure
that no pixel is missed. It is slower, but looks much better and has
a much high stability while moving.
High quality also takes an additional horizontal sample the width of the
horizontal blur matches the height of the vertical blur.
As of clang-format 6.0.1, putting the `/* clang-format off */` hint
around our "invalid" `[vertex]` and `[shader]` statements isn't enough
to prevent a bogus indent of the next comments and first valid statement,
so we need to enclose that first valid statement in the unformatted chunk.
**ERROR**: EffectBlurShaderGLES3: Fragment Program Compilation Failed:
23:191: S0001: Type mismatch, cannot convert from 'int' to 'float'
At: drivers/gles3/shader_gles3.cpp:168:_display_error_with_code() - EffectBlurShaderGLES3: Fragment Program Compilation Failed:
