Issue #1490 - Crash when flattening an XCF file

The default stack size for new threads on MacOS is 512 KiB, making
our 512 KiB limit for stack-allocated buffers in
gimp_operation_layer_mode_real_process() too high.  Lower it to
256 KiB.

(cherry picked from commit 367399e5c0)

app/operations/layer-modes/gimpoperationlayermode.c

@@ -37,7 +37,7 @@
 /* the maximum number of samples to process in one go.  used to limit
  * the size of the buffers we allocate on the stack.
  */
-#define GIMP_COMPOSITE_BLEND_MAX_SAMPLES ((1 << 19) /* 0.5 MiB */  /      \
+#define GIMP_COMPOSITE_BLEND_MAX_SAMPLES ((1 << 18) /* 256 KiB */  /      \
                                           16 /* bytes per pixel */ /      \
                                           2  /* max number of buffers */)