Improve the package

• Gamescope always uses its custom versions of wlroots and never uses the system one. I made it explicitly visible in force_fallback_for (as in upstream).
• Added back transitive dependencies which were lost due to the previous step.
• System glm is also never used because of wrong version (Arch package is 1.0.0, gamescope wants old 0.9.9.8), so also added into force_fallback_for.
• System libliftoff has lower version than required, so also added into force_fallback_for.
• System libraries which can be used made explicitly visible by removing respective submodules.
• Adapted the license to use SPDX scheme.
• Dropped the old unused patch file.