[gl] Fix the number of texture units

[kvark]

Fixes the stack overflow in tests PR checklist:

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

[kvark]

bors r+

[bors[bot]]

Build succeeded:

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