Fix integer overflow resulting in insufficient heap allocation

IMAGE_DIMENSIONS_OK ensures that image width and height are less then
46340, so that maximum number of pixels is ~2**31.

Unfortunately, there are a lot of code that allocates image data with
something like

   malloc(w * h * sizeof(DATA32));

Obviously, on 32-bit machines this results in integer overflow,
insufficient heap allocation, with [massive] out-of-bounds heap
overwrite.
Either X_MAX should be reduced to 32767, or (w)*(h) should be checked to
not exceed ULONG_MAX/sizeof(DATA32).

Security implications:
*) for 32-bit machines: insufficient heap allocation and heap overwrite
in many image loaders, with escalation potential to remote code
execution;
*) for 64-bit machines: it seems, no impact.

src/lib/image.h

@@ -188,7 +188,8 @@ void                __imlib_SaveImage(ImlibImage * im, const char *file,
 
 /* The maximum pixmap dimension is 65535. */
 /* However, for now, use 46340 (46340^2 < 2^31) to avoid buffer overflow issues. */
-#define X_MAX_DIM 46340
+/* Reduced further to 32767, so that (w * h * sizeof(DATA32)) won't exceed ULONG_MAX */
+#define X_MAX_DIM 32767
 
 #define IMAGE_DIMENSIONS_OK(w, h) \
    ( ((w) > 0) && ((h) > 0) && ((w) < X_MAX_DIM) && ((h) < X_MAX_DIM) )