vulkan: lower maximum slab size from 512 MB to 256 MB

AMD recommends 256 MB sized heap allocations. There is some concern here
as to whether or not we're making optimal use for e.g. 4K sized
resources, but it shouldn't matter much for us. (256 MB fits slightly
more than four 4K rgba16f frames, so 512 MB fits slightly more than
eight 4K frames)

src/vulkan/malloc.c

@@ -38,8 +38,8 @@
 // Controls the maximum slab size, to reduce the effect of unbounded slab
 // growth exhausting memory. If the application needs a single allocation
 // that's bigger than this value, it will be allocated directly from the
-// device. (Default: 512 MB)
-#define PLVK_HEAP_MAXIMUM_SLAB_SIZE (1 << 29)
+// device. (Default: 256 MB)
+#define PLVK_HEAP_MAXIMUM_SLAB_SIZE (1 << 28)
 
 // Controls the minimum free region size, to reduce thrashing the free space
 // map with lots of small buffers during uninit. (Default: 1 KB)