MVKPhysicalDevice: Enable shaderResourceMinLod on iOS.

This was actually added in iOS 13, but it wasn't present in the betas.
Since the betas also didn't support family 6, this leads me to suspect
that `min_lod_clamp()` requires family 6. So to be safe, only enable the
feature on family 6.

Update SPIRV-Cross to pull in the changes needed for this.

ExternalRevisions/SPIRV-Cross_repo_revision

@@ -1 +1 @@
-46bf1e99d6e7bbbee51f412e9a97b4ca6bc858b8
+b3c59263a0e8f921df15757c7b28407306a16830

MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm

@@ -1541,6 +1541,10 @@ void MVKPhysicalDevice::initFeatures() {
 	if (supportsMTLFeatureSet(iOS_GPUFamily5_v1)) {
 		_features.multiViewport = true;
 	}
+
+	if (supportsMTLGPUFamily(Apple6)) {
+        _features.shaderResourceMinLod = true;
+	}
 #endif
 
 #if MVK_MACOS