sixel: decgra: set max-non-empty-row-no when resizing the image

This ensures we don’t trim off bottom rows in unhook().

This could happen either because the application used “Set Raster
Attributes” to configure an image size larger than the sixels later
emitted.

Or, the last sixel row contains empty pixel rows.

In either case, the size set with “Set Raster Attributes” defines
the *minimum* image size; the image may still end up being larger.

sixel.c

@@ -1145,6 +1145,7 @@ decgra(struct terminal *term, uint8_t c)
             ph <= term->sixel.max_height && pv <= term->sixel.max_width)
         {
             resize(term, ph, pv);
+            term->sixel.max_non_empty_row_no = pv - 1;
         }
 
         term->sixel.state = SIXEL_DECSIXEL;