This update for prboom-plus fixes the following issues:

Update to snapshot 2.6um+git81 (15431b3a9aac)

* Fix sky scaling for non-standard sky sizes
* Support widescreen assets
* Add OpenGL sprite fuzz options

- Switch to Oelcker's prboom-plus-um code base
- Update to release 2.6um and snapshot 2.6um+git69 (07925d634)

* Support for UMAPINFO lump, customizable intermissions,
customizable episode menu.
* It is now possible to set individual key bindings to
unassigned (completely disable a hotkey function)
* Support for Compressed ZDoom BSP nodes.
* 100 extra sprites (SP00..SP99) for use in DEH/BEX,
200 extra sounds for use in DEH/BEX.
* Add -statdump parameter from Chocolate Doom.
* Add -stroller parameter.
* Added new BEX property "Dropped item" for mobjs.
* New demo format with "PR+UM" signature.
* Add a "-pistolstart" command line parameter.
* Add ALSA sequencer API backend for ALSA MIDI support.
* Now defaults to ffmpeg for video encoding
(switched from previous oggenc2 & x264).

- Update to snapshot 2.5.1.5 SVN4532

* Move to SDL2
* Added the "cap_fps" config variable
* The Del key can be used to clear key bindings in the setup menu.
* Added "Weapon Attack Alignment" option to align the weapon while
shooting. Possible values: "off" (doom behavior), "horizontal",
"centered" and "bobbing" for bobbing during fire.
* Removed the "Strafe 50 on Turns" and "Two-key strafe50:
StrafeOn + MoveLR = strafe50" feature.