[rmodels] Separate GLTF roughness and metallic channels on load (#4739)

Author: Not-Nik

src/rmodels.c

@@ -5347,7 +5347,33 @@ static Model LoadGLTF(const char *fileName)
                     Image imMetallicRoughness = LoadImageFromCgltfImage(data->materials[i].pbr_metallic_roughness.metallic_roughness_texture.texture->image, texPath);
                     if (imMetallicRoughness.data != NULL)
                     {
-                        model.materials[j].maps[MATERIAL_MAP_ROUGHNESS].texture = LoadTextureFromImage(imMetallicRoughness);
+                        Image imMetallic, imRoughness;
+
+                        imMetallic.data = RL_MALLOC(imMetallicRoughness.width * imMetallicRoughness.height);
+                        imRoughness.data = RL_MALLOC(imMetallicRoughness.width * imMetallicRoughness.height);
+
+                        imMetallic.width = imRoughness.width = imMetallicRoughness.width;
+                        imMetallic.height = imRoughness.height = imMetallicRoughness.height;
+
+                        imMetallic.format = imRoughness.format = PIXELFORMAT_UNCOMPRESSED_GRAYSCALE;
+                        imMetallic.mipmaps = imRoughness.mipmaps = 1;
+
+                        for (int x = 0; x < imRoughness.width; x++) {
+                            for (int y = 0; y < imRoughness.height; y++) {
+                                Color color = GetImageColor(imMetallicRoughness, x, y);
+
+                                Color roughness = (Color) {color.g};
+                                Color metallic = (Color) {color.b};
+
+                                ((unsigned char *)imRoughness.data)[y*imRoughness.width + x] = color.g;
+                                ((unsigned char *)imMetallic.data)[y*imMetallic.width + x] = color.b;
+                            }
+                        }
+                        model.materials[j].maps[MATERIAL_MAP_ROUGHNESS].texture = LoadTextureFromImage(imRoughness);
+                        model.materials[j].maps[MATERIAL_MAP_METALNESS].texture = LoadTextureFromImage(imMetallic);
+
+                        UnloadImage(imRoughness);
+                        UnloadImage(imMetallic);
                         UnloadImage(imMetallicRoughness);
                     }