Add fallbacks and mention non-working features when using Compatibility or the Web platform (#1132)

- Make 3D lights with shadows darker when using Compatibility to better
  match the appearance of Forward+/Mobile, due to the use of sRGB blending
  for those lights.
- Silence some warnings when running projects with the Compatibility
  rendering method, such as the one about 2D MSAA not being supported.
- Rebake lightmaps in Global Illumination to improve quality and reduce
  light leaking with improvements from 4.3.
  - Increase probe density for dynamic objects, and mark the moving box
    as a dynamic object for GI purposes.

This is done to improve the experience with the web-based demos,
which always run with the Compatibility rendering method.

3d/material_testers/tester.gd

@@ -27,6 +27,11 @@ var backgrounds: Array[Dictionary] = [
 @onready var camera: Camera3D = $CameraHolder/RotationX/Camera
 
 func _ready() -> void:
+	if RenderingServer.get_current_rendering_method() == "gl_compatibility":
+		# Tweak scene brightness to better match Forward+/Mobile.
+		$WorldEnvironment.environment.tonemap_mode = Environment.TONE_MAPPER_FILMIC
+		$WorldEnvironment.environment.background_energy_multiplier = 2.0
+
 	for background in backgrounds:
 		get_node(^"UI/Background").add_item(background.name)