vulkan/malloc: bump up maximum slab age

Helps avoid GC/realloc thrashing on high FPS displays, especially with
the fact that mpv now calls *both* pl_gpu_flush() and
pl_swapchain_submit(), leading to two GC age ticks per frame.

Probably a more principled solution would be better (e.g. wallclock
age), but for now this is a quick improvement to the status quo.

src/vulkan/malloc.c

@@ -49,7 +49,7 @@
 
 // How long to wait before garbage collecting empty slabs. Slabs older than
 // this many invocations of `vk_malloc_garbage_collect` will be released.
-#define MAXIMUM_SLAB_AGE 8
+#define MAXIMUM_SLAB_AGE 32
 
 // A single slab represents a contiguous region of allocated memory. Actual
 // allocations are served as pages of this. Slabs are organized into pools,