On some hardware, modifying gl_FragColor multiple times can cause large performance drops. This PR writes to a standard temporary variable instead, and copies across to gl_FragColor once only at the end of the fragment shader.
This could potentially lead to large gains in performance on affected hardware.
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:
