[Impeller] Metal:Reset Encoder viewport and scissor rect in case the command specifies no opinion (flutter#34252)

Author: ColdPaleLight

impeller/renderer/backend/metal/render_pass_mtl.mm

@@ -429,28 +429,28 @@ static bool Bind(PassBindingsCache& pass,
                                        : MTLWindingCounterClockwise];
     [encoder setCullMode:ToMTLCullMode(command.cull_mode)];
     [encoder setStencilReferenceValue:command.stencil_reference];
-    if (command.viewport.has_value()) {
-      auto v = command.viewport.value();
-      MTLViewport viewport = {
-          .originX = v.rect.origin.x,
-          .originY = v.rect.origin.y,
-          .width = v.rect.size.width,
-          .height = v.rect.size.height,
-          .znear = v.depth_range.z_near,
-          .zfar = v.depth_range.z_far,
-      };
-      [encoder setViewport:viewport];
-    }
-    if (command.scissor.has_value()) {
-      auto s = command.scissor.value();
-      MTLScissorRect scissor = {
-          .x = static_cast<NSUInteger>(s.origin.x),
-          .y = static_cast<NSUInteger>(s.origin.y),
-          .width = static_cast<NSUInteger>(s.size.width),
-          .height = static_cast<NSUInteger>(s.size.height),
-      };
-      [encoder setScissorRect:scissor];
-    }
+
+    auto v = command.viewport.value_or<Viewport>(
+        {.rect = Rect::MakeSize(Size(GetRenderTargetSize()))});
+    MTLViewport viewport = {
+        .originX = v.rect.origin.x,
+        .originY = v.rect.origin.y,
+        .width = v.rect.size.width,
+        .height = v.rect.size.height,
+        .znear = v.depth_range.z_near,
+        .zfar = v.depth_range.z_far,
+    };
+    [encoder setViewport:viewport];
+
+    auto s = command.scissor.value_or(IRect::MakeSize(GetRenderTargetSize()));
+    MTLScissorRect scissor = {
+        .x = static_cast<NSUInteger>(s.origin.x),
+        .y = static_cast<NSUInteger>(s.origin.y),
+        .width = static_cast<NSUInteger>(s.size.width),
+        .height = static_cast<NSUInteger>(s.size.height),
+    };
+    [encoder setScissorRect:scissor];
+
     if (!bind_stage_resources(command.vertex_bindings, ShaderStage::kVertex)) {
       return false;
     }