chore(assets): VRAM-compress arena textures, BC7 for the nebula sky

Move the nebula sky, planet surface and particle glow textures to VRAM
compression with mipmaps, so they stop being uploaded uncompressed.

The sky panorama is forced to high quality (BC7). It is a 5 MB image of
smooth nebula gradients seen through the arena walls, and BC1/BC3's 5:6:5
endpoint interpolation bands visibly across exactly that kind of content.

Game/assets/models/nebula_planet_planet_surface.png.import

@@ -3,9 +3,10 @@
 importer="texture"
 type="CompressedTexture2D"
 uid="uid://cr3feyv7yldjc"
-path="res://.godot/imported/nebula_planet_planet_surface.png-4b68cca75d2d22d25510c7eb277c0b85.ctex"
+path.s3tc="res://.godot/imported/nebula_planet_planet_surface.png-4b68cca75d2d22d25510c7eb277c0b85.s3tc.ctex"
 metadata={
-"vram_texture": false
+"imported_formats": ["s3tc_bptc"],
+"vram_texture": true
 }
 generator_parameters={
 "md5": "44a2e26010b529339e264deecde957e0"
@@ -14,11 +15,11 @@ generator_parameters={
 [deps]
 
 source_file="res://assets/models/nebula_planet_planet_surface.png"
-dest_files=["res://.godot/imported/nebula_planet_planet_surface.png-4b68cca75d2d22d25510c7eb277c0b85.ctex"]
+dest_files=["res://.godot/imported/nebula_planet_planet_surface.png-4b68cca75d2d22d25510c7eb277c0b85.s3tc.ctex"]
 
 [params]
 
-compress/mode=0
+compress/mode=2
 compress/high_quality=false
 compress/lossy_quality=0.7
 compress/uastc_level=0
@@ -40,4 +41,4 @@ process/normal_map_invert_y=false
 process/hdr_as_srgb=false
 process/hdr_clamp_exposure=false
 process/size_limit=0
-detect_3d/compress_to=1
+detect_3d/compress_to=0