fix texture per-element size

Author: minggo

cocos/renderer/backend/Texture.cpp

@@ -9,13 +9,15 @@ namespace
         uint8_t ret = 0;
         switch (textureFormat)
         {
-            case TextureFormat::RGBA4444:
             case TextureFormat::R8G8B8A8:
                 ret = 4;
                 break;
             case TextureFormat::R8G8B8:
                 ret = 3;
                 break;
+            case TextureFormat::RGBA4444:
+                ret = 2;
+                break;
             case TextureFormat::A8:
                 ret = 1;
                 break;

cocos/renderer/backend/metal/TextureMTL.mm

@@ -102,9 +102,12 @@ bool convertData(uint8_t* src, unsigned int length, TextureFormat format, uint8_
     createTexture(mtlDevice, descriptor);
     createSampler(mtlDevice, descriptor);
 
-    // Metal doesn't support RGB888, so should convert to RGBA888;
-    if (TextureFormat::R8G8B8 == _textureFormat)
+    // Metal doesn't support RGB888/RGBA4444, so should convert to RGBA888;
+    if (TextureFormat::R8G8B8 == _textureFormat ||
+        TextureFormat::RGBA4444 == _textureFormat)
+    {
         _bytesPerElement = 4;
+    }
 
     _bytesPerRow = _bytesPerElement * descriptor.width;
 }