Fix glow visual compatibility regression (issue #112469)

Reverts the default value of Environment.glow_hdr_threshold from 0.0
back to 1.0 to restore the expected glow appearance in existing projects.

The default was inadvertently changed from 1.0 to 0.0 in PR #110077,
which caused glow effects to render dramatically different across all
rendering methods (Forward+, Mobile, and GL Compatibility). This broke
backward compatibility with existing projects like the Kenney 3D
Platformer starter kit.

Changed files:
- scene/resources/environment.h
- servers/rendering/storage/environment_storage.h
- drivers/gles3/effects/glow.h
- drivers/gles3/rasterizer_scene_gles3.cpp
- doc/classes/Environment.xml

Setting the value back to 1.0 aligns with documented recommendations
and restores visual consistency.

Fixes #112469

scene/resources/environment.h

@@ -168,7 +168,7 @@ private:
 	float glow_mix = 0.05;
 	float glow_bloom = 0.0;
 	GlowBlendMode glow_blend_mode = GLOW_BLEND_MODE_SCREEN;
-	float glow_hdr_bleed_threshold = 0.0;
+	float glow_hdr_bleed_threshold = 1.0;
 	float glow_hdr_bleed_scale = 2.0;
 	float glow_hdr_luminance_cap = 12.0;
 	float glow_map_strength = 0.8f;