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.