fate: add coverage for background disposal in gif decoder

Signed-off-by: Paul B Mahol <onemda@gmail.com>

tests/fate/gif.mak

 FATE_GIF += fate-gif-color
 fate-gif-color: CMD = framecrc -i $(SAMPLES)/gif/tc217.gif -pix_fmt bgra
 
+FATE_GIF += fate-gif-disposal-background
+fate-gif-disposal-background: CMD = framecrc -trans_color 0 -i $(SAMPLES)/gif/m4nb.gif -pix_fmt bgra
+
 FATE_GIF += fate-gif-disposal-restore
 fate-gif-disposal-restore: CMD = framecrc -i $(SAMPLES)/gif/banner2.gif -pix_fmt bgra
 

tests/ref/fate/gif-disposal-background

+#tb 0: 1/100
+0,          0,          0,        1,    10368, 0x8b200cc8
+0,         50,         50,        1,    10368, 0x4e208e4f
+0,        100,        100,        1,    10368, 0x044dba8b
+0,        150,        150,        1,    10368, 0x1d617e09
+0,        200,        200,        1,    10368, 0xae38a6d2