Fix blur glow

[arukagiri]

Per the migration to Godot 4 tracker issue post here is the fix for the blur glow shader.

Issues found (and fixed):

• Redeclaration of TAU constant in blur shader.
• Shader not flipping texture on y anymore.
• I am not fully sure exactly what, but the subviewports where not showing correctly in runtime, something to do with the size of the viewport not scaling as expected, whereas it was fine on the editor. Fix might not be proper but I just changed the anchors of the viewport container.

[NathanLovato]

Thanks much for the contributions! They're much appreciated.