More crash fixes, restore Metal support

Cocoa/GBOpenGLView.m

@@ -13,12 +13,14 @@
     double scale = self.window.backingScaleFactor;
     glViewport(0, 0, self.bounds.size.width * scale, self.bounds.size.height * scale);
     
+    if (gbview.gb) {
         [self.shader renderBitmap:gbview.currentBuffer
                          previous:gbview.frameBlendingMode? gbview.previousBuffer : NULL
                             sized:NSMakeSize(GB_get_screen_width(gbview.gb), GB_get_screen_height(gbview.gb))
                            inSize:self.bounds.size
                             scale:scale
                  withBlendingMode:gbview.frameBlendingMode];
+    }
     glFlush();
 }
 

Cocoa/GBViewMetal.m

@@ -23,7 +23,7 @@ static const vector_float2 rect[] =
 + (bool)isSupported
 {
     if (MTLCopyAllDevices) {
-        return false; //[MTLCopyAllDevices() count];
+        return [MTLCopyAllDevices() count];
     }
     return false;
 }