metal use MTLPixelFormatDepth16Unorm for D16 format (#3246)

src/renderer_mtl.mm

@@ -329,7 +329,7 @@ namespace bgfx { namespace mtl
 		{ MTLPixelFormatRGB10A2Unorm,                   MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, true  }, // RGB10A2
 		{ MTLPixelFormatRG11B10Float,                   MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, true  }, // RG11B10F
 		{ MTLPixelFormatInvalid,                        MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, false }, // UnknownDepth
-		{ MTLPixelFormatDepth32Float,                   MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, false }, // D16
+		{ MTLPixelFormatDepth16Unorm,                   MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, false }, // D16
 		{ MTLPixelFormatDepth32Float,                   MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, false }, // D24
 		{ MTLPixelFormat(255/*Depth24Unorm_Stencil8*/), MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, false }, // D24S8
 		{ MTLPixelFormatDepth32Float,                   MTLPixelFormatInvalid,                       MTLReadWriteTextureTierNone, { $R, $G, $B, $A }, false }, // D32
@@ -687,6 +687,10 @@ BX_STATIC_ASSERT(BX_COUNTOF(s_accessNames) == Access::Count, "Invalid s_accessNa
 				: 0
 				;
 
+			if (!(iOSVersionEqualOrGreater("13.0.0") || macOSVersionEqualOrGreater(12, 0, 0))){
+				s_textureFormat[TextureFormat::D16].m_fmt = MTLPixelFormatDepth32Float;
+			}
+
 			for (uint32_t ii = 0; ii < TextureFormat::Count; ++ii)
 			{
 				uint16_t support = 0;