Get the Quake situation under control

Changelog:

• updated DarkPlaces engine, moved old version, but tread with caution because the new version can be buggy, that's why there hasn't been a release for so long
• the theoretical version of DarkPlaces is derived from the date when the related git commit was made, and because there is no release associated with it, ebuilds need to declare it as a pre-release
• DarkPlaces 20170829_beta1 and Quake 1.5 have been merged into a single package to make sure that the Quake 1.5 experience is delivered by src_prepare-overlay, but does not coincide with regular Quake gameplay and Quake mods
• USE flags of both packages have been expanded to include configurable support for IPv6, CD/OST playback (CD support in DarkPlaces automatically pulls in OGG support in case a CDDA rip exists, upstream-defined behavior), video recording, and de-selectable SSE and SSE2 optimizations on x86
• all affected packages were playtested on the following systems:
• • Ryzen 2700 w/ Radeon 6400 (amdgpu)
• • Raspberry Pi 4B 8GB (vc4)
• • Apple PowerMac11,2 G5 w/ GeForce 6600 (nouveau)
• • dual-socket Pentium 3 VM w/ QXL (qxl)
• the results of those playtests indicate that DarkPlaces and furthermore Quake 1.5 work fine on amd64, x86 and arm64
• these playtests unearthed regressions for big endian CPUs in DarkPlaces that in best case scenario produce inconvenient artifacts (anti-light radius of 3m surrounding player) and in worst case scenario will crash the game when PNG textures are used due to incorrect readings from file headers and therefore allocations of ridiculous amounts of memory (8GB) for tiny files (200K)
• as a summary of these playtests, I think it's justified to give these 2 packages ~amd64, ~arm64 and ~x86 keywords
• all pkgcheck errors for these packages are now fixed