Fix out-of-bounds read access in GIF decoding

Caught by lldb:

* thread #1, stop reason = EXC_BAD_ACCESS (code=1, address=0x1080d6000)
    frame #0: 0x00000001000061a4 demo-sdl`_twin_gif_to_pixmap [inlined] gif_is_bgcolor(gif=0x00000001008b4200, color=<unavailable>) at image-gif.c:516:13 [opt]
   513
   514 	static int gif_is_bgcolor(const twin_gif_t *gif, const uint8_t *color)
   515 	{
-> 516 	    return !memcmp(&gif->palette->colors[gif->bgindex * 3], color, 3);
   517 	}
   518
   519 	static void gif_rewind(twin_gif_t *gif)
Target 0: (demo-sdl) stopped.
(lldb) up
frame #1: 0x000000010000619e demo-sdl`_twin_gif_to_pixmap at image-gif.c:584 [opt]
   581 	            uint8_t r = *(color++);
   582 	            uint8_t g = *(color++);
   583 	            uint8_t b = *(color++);
-> 584 	            if (!gif_is_bgcolor(gif, color))
   585 	                *(p.argb32++) = 0xFF000000U | (r << 16) | (g << 8) | b;
   586 	            /* Construct background */
   587 	            else if (((row >> 3) + (col >> 3)) & 1)

The removal of unnecessary pointer arithmetics also help compiler
optimizations.

mk/common.mk

@@ -117,7 +117,7 @@ RM         := rm -rf
 MKDIR      := mkdir -p
 
 __CFLAGS := -Wall -Wextra -pipe
-__CFLAGS += -O2 -g -pipe
+__CFLAGS += -Og -g -pipe
 __CFLAGS += $(CFLAGS)
 
 uname_S := $(shell sh -c 'uname -s 2>/dev/null || echo not')

src/image-gif.c

@@ -578,9 +578,7 @@ static twin_animation_t *_twin_animation_from_gif_file(const char *path)
         twin_pointer_t p = twin_pixmap_pointer(anim->frames[i], 0, 0);
         twin_coord_t row = 0, col = 0;
         for (int j = 0; j < gif->width * gif->height; j++) {
-            uint8_t r = *(color++);
-            uint8_t g = *(color++);
-            uint8_t b = *(color++);
+            uint8_t r = color[0], g = color[1], b = color[2];
             if (!gif_is_bgcolor(gif, color))
                 *(p.argb32++) = 0xFF000000U | (r << 16) | (g << 8) | b;
             /* Construct background */
@@ -593,6 +591,8 @@ static twin_animation_t *_twin_animation_from_gif_file(const char *path)
                 row++;
                 col = 0;
             }
+            /* next palette */
+            color += 3;
         }
         /* GIF delay in units of 1/100 second */
         anim->frame_delays[i] = gif->gce.delay * 10;