This avoids allocating a potentially large array in case just the first
device is needed. Originally I did this in a hope to avoid a stall + CPU
power management issues due to some bad shit in the AMD driver, but it
seems that enumerating even just one device still makes it stall. Sigh.
Similarly to Corrade Assert.h, so it's possible to include this header
without having to worry about irrelevant overhead when asserts are
disabled.
Also test it properly, as it should have been from the start.
I'm a bit unsure why there's a device extension that actually gets used
on an instance and doesn't need any enabling (and thus there's
currently no way to disable it to test all code paths, hmm).
Memory type flags are put into a new, separate Memory.h header as those
will be needed more often than the (ever-growing) DeviceProperties --
from Image and Buffer constructors, in particular.