Add an import option to force TextureArrays as normal maps for RGTC

i have added the option to chose normal map for channel pack. this will force the to use red and green channels for the texture only. the option is added meaning it will not conflict with the existing settings.

i have made this change as a response to my own proposal

Bugsquad edit: Closes godotengine/godot-proposals#3184

editor/import/resource_importer_layered_texture.cpp

@@ -138,7 +138,7 @@ void ResourceImporterLayeredTexture::get_import_options(List<ImportOption> *r_op
 	r_options->push_back(ImportOption(PropertyInfo(Variant::FLOAT, "compress/lossy_quality", PROPERTY_HINT_RANGE, "0,1,0.01"), 0.7));
 	r_options->push_back(ImportOption(PropertyInfo(Variant::INT, "compress/hdr_compression", PROPERTY_HINT_ENUM, "Disabled,Opaque Only,Always"), 1));
 	r_options->push_back(ImportOption(PropertyInfo(Variant::INT, "compress/bptc_ldr", PROPERTY_HINT_ENUM, "Disabled,Enabled,RGBA Only"), 0));
-	r_options->push_back(ImportOption(PropertyInfo(Variant::INT, "compress/channel_pack", PROPERTY_HINT_ENUM, "sRGB Friendly,Optimized"), 0));
+	r_options->push_back(ImportOption(PropertyInfo(Variant::INT, "compress/channel_pack", PROPERTY_HINT_ENUM, "sRGB Friendly,Optimized,Normal Map (RG channels"), 0));
 	r_options->push_back(ImportOption(PropertyInfo(Variant::BOOL, "mipmaps/generate"), true));
 	r_options->push_back(ImportOption(PropertyInfo(Variant::INT, "mipmaps/limit", PROPERTY_HINT_RANGE, "-1,256"), -1));
 
@@ -249,7 +249,7 @@ void ResourceImporterLayeredTexture::_save_tex(Vector<Ref<Image>> p_images, cons
 			}
 
 			if (p_mipmaps) {
-				p_images.write[i]->generate_mipmaps();
+				p_images.write[i]->generate_mipmaps(p_csource == Image::COMPRESS_SOURCE_NORMAL);
 			} else {
 				p_images.write[i]->clear_mipmaps();
 			}
@@ -356,6 +356,9 @@ Error ResourceImporterLayeredTexture::import(const String &p_source_file, const
 	Image::CompressSource csource = Image::COMPRESS_SOURCE_GENERIC;
 	if (channel_pack == 0) {
 		csource = Image::COMPRESS_SOURCE_SRGB;
+	} else if (channel_pack == 2) {
+		// force normal
+		csource = Image::COMPRESS_SOURCE_NORMAL;
 	}
 
 	Image::UsedChannels used_channels = image->detect_used_channels(csource);