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

doc/classes/Environment.xml

@@ -143,7 +143,7 @@
 		<member name="glow_hdr_scale" type="float" setter="set_glow_hdr_bleed_scale" getter="get_glow_hdr_bleed_scale" default="2.0">
 			The bleed scale of the HDR glow.
 		</member>
-		<member name="glow_hdr_threshold" type="float" setter="set_glow_hdr_bleed_threshold" getter="get_glow_hdr_bleed_threshold" default="0.0">
+		<member name="glow_hdr_threshold" type="float" setter="set_glow_hdr_bleed_threshold" getter="get_glow_hdr_bleed_threshold" default="1.0">
 			The lower threshold of the HDR glow. When using the Mobile rendering method (which only supports a lower dynamic range up to [code]2.0[/code]), this may need to be below [code]1.0[/code] for glow to be visible. A value of [code]0.9[/code] works well in this case. This value also needs to be decreased below [code]1.0[/code] when using glow in 2D, as 2D rendering is performed in SDR.
 		</member>
 		<member name="glow_intensity" type="float" setter="set_glow_intensity" getter="get_glow_intensity" default="0.3">