sprites: Tweak horizontal offsets for zombie.

Vanilla Doom doesn't flip X offsets properly when mirroring sprites,
so adjust mirrored sprites so that the monster is centered within
each frame and mirrors correctly.

These are small tweaks and I've only done frames A-D so far.

buildcfg.txt

@@ -1772,25 +1772,25 @@ BOSSL0	34	79
 BOSSM0	34	79
 BOSSN0	34	79
 BOSSO0	34	79
-POSSA1		18	50			; former human
-POSSA2A8	17	50
-POSSA3A7	21	50
-POSSA4A6	21	48
-POSSA5		21	47
-POSSB1		19	52
-POSSB2B8	18	51
-POSSB3B7	18	52
-POSSB4B6	20	52
-POSSB5		18	51
-POSSC1		17	50
-POSSC2C8	17	50
-POSSC3C7	18	52
-POSSC4C6	20	51
+POSSA1		17	50			; former human
+POSSA2A8	20	50
+POSSA3A7	20	50
+POSSA4A6	26	48
+POSSA5		17	47
+POSSB1		18	52
+POSSB2B8	20	51
+POSSB3B7	22	52
+POSSB4B6	28	52
+POSSB5		17	51
+POSSC1		19	50
+POSSC2C8	19	50
+POSSC3C7	21	52
+POSSC4C6	30	51
 POSSC5		17	51
-POSSD1		16	52
-POSSD2D8	18	51
-POSSD3D7	21	52
-POSSD4D6	22	52
+POSSD1		19	52
+POSSD2D8	19	51
+POSSD3D7	23	52
+POSSD4D6	26	52
 POSSD5		19	51
 POSSE1		12	50
 POSSE2E8	21	50