Fix crash in Metal backend with screen-space refractions (#2104)

filament/backend/src/metal/MetalDriver.mm

@@ -850,6 +850,9 @@ void MetalDriver::blit(TargetBufferFlags buffers,
     id<MTLTexture> srcTexture = srcTarget->getBlitColorSource();
     id<MTLTexture> dstTexture = dstTarget->getColor();
 
+    ASSERT_PRECONDITION(srcTexture != nil && dstTexture != nil,
+            "Source texture and destination texture must not be nil");
+
     // Metal's texture coordinates have (0, 0) at the top-left of the texture, but Filament's
     // coordinates have (0, 0) at bottom-left.
     MTLRegion srcRegion = MTLRegionMake2D(

filament/backend/src/metal/MetalHandles.mm

@@ -441,6 +441,9 @@ id<MTLTexture> MetalRenderTarget::getDepthResolve() {
 }
 
 id<MTLTexture> MetalRenderTarget::getBlitColorSource() {
+    if (defaultRenderTarget) {
+        return acquireDrawable(context);
+    }
     if (color) {
         return color;
     }
@@ -448,6 +451,9 @@ id<MTLTexture> MetalRenderTarget::getBlitColorSource() {
 }
 
 id<MTLTexture> MetalRenderTarget::getBlitDepthSource() {
+    if (defaultRenderTarget) {
+        return acquireDepthTexture(context);
+    }
     if (depth) {
         return depth;
     }