[mtl] adjust render pass compatibility check at the start of a pass

[kvark]

Fixes #3509 This was a regression from #3486 PR checklist:

• [x] make succeeds (on *nix)
• [x] make reftests succeeds
• [x] tested examples with the following backends: metal

[kvark]

bors r+

[bors[bot]]

Build succeeded:

• iOS Stable
• Ubuntu Stable
• Android Stable
• Windows Stable
• Dependency Check
• MacOS Stable