feat: add simple transparency

tests/serialization_tests.cpp

@@ -293,6 +293,8 @@ namespace {
         material.roughnessFactor = 0.55f;
         material.emissiveColor = {1.0f, 0.5f, 0.0f};
         material.textureFilteringMode = TextureFilteringMode::Anisotropic;
+        material.blendMode = BlendMode::Transparent;
+        material.opacity = 0.25f;
         material.diffuseTexture = 3;
         material.emissiveTexture = 7;
         material.normalTexture = 9;
@@ -315,7 +317,8 @@ namespace {
         check(loaded.name == "Crate" && loaded.baseColor == material.baseColor &&
               close(loaded.metallicFactor, 0.3f) && close(loaded.roughnessFactor, 0.55f) &&
               loaded.emissiveColor == material.emissiveColor &&
-              loaded.textureFilteringMode == TextureFilteringMode::Anisotropic,
+              loaded.textureFilteringMode == TextureFilteringMode::Anisotropic &&
+              loaded.blendMode == BlendMode::Transparent && close(loaded.opacity, 0.25f),
               "material factors must round-trip through json");
         check(loaded.diffuseTexture == 30 && loaded.emissiveTexture == 70 &&
               loaded.normalTexture == NULL_IMAGE_ID && loaded.metallicTexture == NULL_IMAGE_ID,