1.44M IMG: Increase gap3 from 84 bytes to 108 bytes

This better matches the disks formatted by IBM PCs, and also matches
the recommended "Format GAP3" in Table 4-1 of
"PC8477B (SuperFDC) Advanced Floppy Disk Controller"
<https://www.ardent-tool.com/datasheets/NS_PC8477B.pdf>

Refs keirf/greaseweazle#568

Author: keirf

src/image/img.c

@@ -102,7 +102,7 @@ const static struct raw_type {
     {  9, _S(2), _IAM, 84, 1, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 720k */
     { 10, _S(2), _IAM, 30, 1, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 800k */
     { 11, _S(2), _IAM,  3, 2, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 880k */
-    { 18, _S(2), _IAM, 84, 1, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 1.44M */
+    { 18, _S(2), _IAM,108, 1, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 1.44M */
     { 19, _S(2), _IAM, 70, 1, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 1.52M */
     { 21, _S(2), _IAM, 12, 2, 2, 1, 0, 3, 0, _C(80), _R(300) }, /* 1.68M */
     { 20, _S(2), _IAM, 40, 1, 2, 1, 0, 0, 0, _C(80), _R(300) }, /* 1.6M */