Improve Environment color adjustments; specifically brightness and HDR 2D contrast.

This commit changes adjustments to behave as follows for all rendering configurations:

- Apply brightness to linear-encoded values, preventing contrast, saturation, and hue from being affected.
- Apply contrast to perceptually uniform (nonlinear sRGB-encoded) values, matching existing behavior when HDR 2D is disabled and producing optimal visual quality.
- Apply saturation with even color channel weights. This causes brightness of certain colors to change, but matches existing behavior when HDR 2D is disabled.

Adjustments are applied after glow and tonemapping to match existing behavior.

doc/classes/Environment.xml

@@ -36,19 +36,19 @@
 	</methods>
 	<members>
 		<member name="adjustment_brightness" type="float" setter="set_adjustment_brightness" getter="get_adjustment_brightness" default="1.0">
-			The global brightness value of the rendered scene. Effective only if [member adjustment_enabled] is [code]true[/code].
+			Applies a simple brightness adjustment to the rendered image after tonemaping. To adjust scene brightness use [member tonemap_exposure] instead, which is applied before tonemapping and thus less prone to issues with bright colors. Effective only if [member adjustment_enabled] is [code]true[/code].
 		</member>
 		<member name="adjustment_color_correction" type="Texture" setter="set_adjustment_color_correction" getter="get_adjustment_color_correction">
 			The [Texture2D] or [Texture3D] lookup table (LUT) to use for the built-in post-process color grading. Can use a [GradientTexture1D] for a 1-dimensional LUT, or a [Texture3D] for a more complex LUT. Effective only if [member adjustment_enabled] is [code]true[/code].
 		</member>
 		<member name="adjustment_contrast" type="float" setter="set_adjustment_contrast" getter="get_adjustment_contrast" default="1.0">
-			The global contrast value of the rendered scene (default value is 1). Effective only if [member adjustment_enabled] is [code]true[/code].
+			Increasing [member adjustment_contrast] will make dark values darker and bright values brighter. This simple adjustment is applied to the rendered image after tonemaping. When set to a value greater than [code]1.0[/code], [member adjustment_contrast] is prone to clipping colors that become too bright or too dark. Effective only if [member adjustment_enabled] is [code]true[/code].
 		</member>
 		<member name="adjustment_enabled" type="bool" setter="set_adjustment_enabled" getter="is_adjustment_enabled" default="false">
 			If [code]true[/code], enables the [code]adjustment_*[/code] properties provided by this resource. If [code]false[/code], modifications to the [code]adjustment_*[/code] properties will have no effect on the rendered scene.
 		</member>
 		<member name="adjustment_saturation" type="float" setter="set_adjustment_saturation" getter="get_adjustment_saturation" default="1.0">
-			The global color saturation value of the rendered scene (default value is 1). Effective only if [member adjustment_enabled] is [code]true[/code].
+			Applies a simple saturation adjustment to the rendered image after tonemaping. When [member adjustment_saturation] is set to [code]0.0[/code], the rendered image will be fully converted to a grayscale image. Effective only if [member adjustment_enabled] is [code]true[/code].
 		</member>
 		<member name="ambient_light_color" type="Color" setter="set_ambient_light_color" getter="get_ambient_light_color" default="Color(0, 0, 0, 1)">
 			The ambient light's [Color]. Only effective if [member ambient_light_sky_contribution] is lower than [code]1.0[/code] (exclusive).