patches/textures: replacement E4 starry sky for Double Impact

Double Impact's original sky texture is derived from an image with
unknown copyright restrictions (see [1]). Construct a replacement from
an image of the globular cluster M80 from the Hubble Space Telescope
(Credits: NASA; public domain; sources [2] [3]) and a random starfield
image generated by ImageMagick (adapted from [4])

Note the original's "dark sky with bright shadow-casting light source"
effect which was built into the maps - this replacement is carefully
tuned to preserve that effect (as any improvement should also do).

[1] http://www.doomworld.com/vb/post/1222877
[2] http://en.wikipedia.org/wiki/File:A_Swarm_of_Ancient_Stars_-_GPN-2000-000930.jpg
[3] http://grin.hq.nasa.gov/ABSTRACTS/GPN-2000-000930.html
[4] http://www.imagemagick.org/Usage/advanced/#stars

Signed-off-by: RjY <rjy@users.sourceforge.net>

patches/sky4a.gif

@@ -0,0 +1 @@
+rjy/sky4a.gif

patches/sky4b.gif

@@ -0,0 +1 @@
+rjy/sky4b.gif

textures/combined.txt

@@ -1935,8 +1935,11 @@ WFALL4			64	128
 
 ; These textures are only in Ultimate Doom
 
-SKY4        		256	128
+SKY4        		1024	128
-*	SKY4       	0	0
+*	SKY4A       	0	0
+*	SKY4A       	256	0
+*	SKY4B       	512	0
+*	SKY4A       	768	0
 
 #endif