Add Threen icon (copied from Tween)

Uses an absolute path for compatibility with 4.1.x.

We can switch to relative paths for both the icon and the libraries once
we start targeting 4.2 as minimal version.

bin/Threen.svg.import

@@ -0,0 +1,37 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+uid="uid://dsk3tvoo4ooj6"
+path="res://.godot/imported/Threen.svg-93a9b1dcd3e634afdd64a494c0aa9265.ctex"
+metadata={
+"vram_texture": false
+}
+
+[deps]
+
+source_file="res://bin/Threen.svg"
+dest_files=["res://.godot/imported/Threen.svg-93a9b1dcd3e634afdd64a494c0aa9265.ctex"]
+
+[params]
+
+compress/mode=0
+compress/high_quality=false
+compress/lossy_quality=0.7
+compress/hdr_compression=1
+compress/normal_map=0
+compress/channel_pack=0
+mipmaps/generate=false
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/fix_alpha_border=true
+process/premult_alpha=false
+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
+svg/scale=1.0
+editor/scale_with_editor_scale=false
+editor/convert_colors_with_editor_theme=false

bin/threen.gdextension

@@ -3,6 +3,10 @@
 entry_symbol = "threen_library_init"
 compatibility_minimum = "4.1"
 
+[icons]
+
+Threen = "res://bin/Threen.svg"
+
 [libraries]
 
 linux.x86_64.debug = "res://bin/libthreen.linux.template_debug.x86_64.so"