Work around what appears to be an integer promotion issue

On an aarch64 Chromebook running gcc 12.2.0, these would wrap in
weird ways causing vector fonts to be positioned incorrectly.

It managed to say that (25 - -7) * 4 / 3 == -168
This casting dance appears to resolve the issue.

src/syncterm/ripper.c

@@ -8654,7 +8654,7 @@ write_char(char ch)
                 // yh = font_metrics[rip.font.num][3].base * mult / div;
                 // This way seems insane... but seems to work.
-		yh = (((char)rip_fonts[rip.font.num - 1][0x88]) - ((char)rip_fonts[rip.font.num - 1][0x8a])) * mult
+		yh = (((signed char)rip_fonts[rip.font.num - 1][0x88]) - ((signed char)rip_fonts[rip.font.num - 1][0x8a])) * mult
 		    / div;
 // printf("Font: %d @ %d\n", rip.font.num, rip.font.size);
@@ -8680,8 +8680,8 @@ write_char(char ch)
 				cx |= 0x80;
 			if (cy & 0x40)
 				cy |= 0x80;
-			bcx = (cx * mult) / div;
-			bcy = (cy * mult) / div;
+			bcx = ((signed char)cx * mult) / div;
+			bcy = ((signed char)cy * mult) / div;
 			if (rip.font.vertical) {
 				dx = yh - bcy;
 				dy = -bcx;