print_img: correct off-center print area via per-device pin_offset

On the PT-P900W the print area is not centered on the 560-pin printhead:
its center sits at pin 272 rather than the head center (pin 280), a fixed
8-pin offset that is constant across all tape widths. Centering the raster
data on the head therefore prints ~0.5mm off-center (measured as a constant
~1mm margin delta across 12/18/36mm tapes, confirmed against the spec).

Add a pin_offset field to _pt_dev_info (0 = centered, the default for all
existing printers) and apply it in print_img. Set it to -8 for the
PT-P900Wc, which reproduces the spec left-margin exactly for every tape
width ((560-print_area)/2 - 8 = documented left margin).

Reference: Brother "Software Developer's Manual - Raster Command Reference,
PT-P900/P900W/P950NW", section 2.3.5 "Raster line" (per-tape left/right
margin and print-area pin counts for the 560-pin head).
https://download.brother.com/welcome/docp100407/cv_ptp900_eng_raster_102.pdf

include/ptouch.h

@@ -57,6 +57,11 @@ struct _pt_dev_info {
 	int dpi;		/* Dots per inch of the printhead */
 	//size_t bytes_per_line;
 	int flags;
+	/* Pins to shift the print area from the centre of the printhead.
+	   On some models the print area is not centred on the head; see the
+	   Brother "Software Developer's Manual - Raster Command Reference,
+	   PT-P900/P900W/P950NW", section 2.3.5 "Raster line". 0 = centred. */
+	int pin_offset;
 };
 typedef struct _pt_dev_info *pt_dev_info;
 

src/libptouch.c

@@ -89,10 +89,14 @@ struct _pt_dev_info ptdevs[] = {
 	{0x04f9, 0x2074, "PT-D600", 128, 180, FLAG_RASTER_PACKBITS},
 	/* PT-D600 was reported to work, but with some quirks (premature
 	   cutting of tape, printing maximum of 73mm length) */
-	{0x04f9, 0x2085, "PT-P900Wc", 560, 360, FLAG_RASTER_PACKBITS|FLAG_P700_INIT|FLAG_USE_INFO_CMD|FLAG_HAS_PRECUT},
+	{0x04f9, 0x2085, "PT-P900Wc", 560, 360, FLAG_RASTER_PACKBITS|FLAG_P700_INIT|FLAG_USE_INFO_CMD|FLAG_HAS_PRECUT, -8},
 	/* PT-P900Wc: 360dpi, 560px printhead (70 bytes/raster line). Verified
 	   printing on 36mm laminated tape. Narrower tape widths use Brother's
-	   documented 360dpi print areas (see tape_info) but are untested. */
+	   documented 360dpi print areas (see tape_info) but are untested.
+	   pin_offset = -8: the print area is centred at pin 272, not the head
+	   centre (pin 280) - a constant offset across all tape widths. See the
+	   Brother "Software Developer's Manual - Raster Command Reference,
+	   PT-P900/P900W/P950NW", section 2.3.5 "Raster line". */
 	{0x04f9, 0x20af, "PT-P710BT", 128, 180, FLAG_RASTER_PACKBITS|FLAG_HAS_PRECUT},
 	{0x04f9, 0x20df, "PT-D410", 128, 180, FLAG_USE_INFO_CMD|FLAG_HAS_PRECUT|FLAG_D460BT_MAGIC},
 	{0x04f9, 0x20e0, "PT-D460BT", 128, 180, FLAG_P700_INIT|FLAG_USE_INFO_CMD|FLAG_HAS_PRECUT|FLAG_D460BT_MAGIC},
@@ -175,6 +179,7 @@ int ptouch_open(ptouch_dev *ptdev)
 				(*ptdev)->devinfo->dpi=ptdevs[k].dpi;
 				(*ptdev)->devinfo->max_px=ptdevs[k].max_px;
 				(*ptdev)->devinfo->flags=ptdevs[k].flags;
+				(*ptdev)->devinfo->pin_offset=ptdevs[k].pin_offset;
 				return 0;
 			}
 		}

src/ptouch-print.c

@@ -186,7 +186,13 @@ int print_img(ptouch_dev ptdev, gdImage *im, int chain, int precut)
 		return -1;
 	}
 	printf(_("image size (%ipx x %ipx)\n"), gdImageSX(im), gdImageSY(im));
-	int offset = ((int)max_pixels / 2) - (gdImageSY(im)/2);	/* always print centered */
+	int offset = ((int)max_pixels / 2) - (gdImageSY(im)/2);	/* center within the printhead */
+	/* Some models do not center the print area on the printhead. pin_offset
+	   is a fixed per-device shift (in pins/px, constant across tape widths)
+	   that corrects for this; it is 0 where centering is already correct.
+	   See the Brother "Software Developer's Manual - Raster Command
+	   Reference, PT-P900/P900W/P950NW", section 2.3.5 "Raster line". */
+	offset += ptdev->devinfo->pin_offset;
 	printf("max_pixels=%ld, offset=%d\n", max_pixels, offset);
 	if ((ptdev->devinfo->flags & FLAG_RASTER_PACKBITS) == FLAG_RASTER_PACKBITS) {
 		if (arguments.debug) {