[gl] remove bogus assertions about viewports

[kvark]

Fixes a misfiring assert I added recently... PR checklist:

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

[kvark]

bors r+

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable