One cannot use #if...#endif within macro calls in Visual Studio and
possibly other compilers, and there are more uses of VLAs that need to be
replaced with g_newa().
There were also checks for the clip type in gskvulkanrenderpass.c which
were possibly not done right (using the address of the type value to check
for a type value), which triggered errors as one is attempting to compare
a pointer type to an enum/int type.
https://bugzilla.gnome.org/show_bug.cgi?id=773299
This way we can pass the command pool around.
And that allows us to allocate and submitcustom buffers.
And that is necessary to make staging images work.
