Fix crashes for Metal 3.0 capabilities.

Don't allocate MTLHeap of zero size.
MVKImage check for device memory when accessing MTLCPUCacheMode..

MoltenVK/MoltenVK/GPUObjects/MVKDeviceMemory.mm

@@ -178,6 +178,9 @@ bool MVKDeviceMemory::ensureMTLHeap() {
 	// Don't bother if we don't have placement heaps.
 	if (!getDevice()->_pMetalFeatures->placementHeaps) { return true; }
 
+	// Can't create MTLHeaps of zero size.
+	if (_allocationSize == 0) { return true; }
+
 #if MVK_MACOS
 	// MTLHeaps on Mac must use private storage for now.
 	if (_mtlStorageMode != MTLStorageModePrivate) { return true; }

MoltenVK/MoltenVK/GPUObjects/MVKImage.h

@@ -216,7 +216,7 @@ public:
     MTLStorageMode getMTLStorageMode();
 
 	/** Returns the Metal CPU cache mode used by this image. */
-	inline MTLCPUCacheMode getMTLCPUCacheMode() { return _deviceMemory->getMTLCPUCacheMode(); }
+	MTLCPUCacheMode getMTLCPUCacheMode();
 
 	/** 
 	 * Returns whether the memory is automatically coherent between device and host. 

MoltenVK/MoltenVK/GPUObjects/MVKImage.mm

@@ -487,6 +487,10 @@ MTLStorageMode MVKImage::getMTLStorageMode() {
     return stgMode;
 }
 
+MTLCPUCacheMode MVKImage::getMTLCPUCacheMode() {
+	return _deviceMemory ? _deviceMemory->getMTLCPUCacheMode() : MTLCPUCacheModeDefaultCache;
+}
+
 bool MVKImage::isMemoryHostCoherent() {
     return (getMTLStorageMode() == MTLStorageModeShared);
 }