When the `WLR_GBM_EXPLICIT_UPGRADE` flag is set to 1, the GBM allocator
will attempt to "upgrade" buffers allocated with implicit modifiers to
be managed with explicit modifiers.
This is useful to enable the conservative modifier fallback for the
Vulkan renderer on drivers/hardware that fully support implicit and
explicit modifiers, including being able to report the modifier of an
implicitly allocated BO.
Stop trying to maintain a per-file _POSIX_C_SOURCE. Instead,
require POSIX.1-2008 globally. A lot of core source files depend
on that already.
Some care must be taken on a few select files where we need a bit
more than POSIX. Some files need XSI extensions (_XOPEN_SOURCE) and
some files need BSD extensions (_DEFAULT_SOURCE). In both cases,
these feature test macros imply _POSIX_C_SOURCE. Make sure to not
define both these macros and _POSIX_C_SOURCE explicitly to avoid
POSIX requirement conflicts (e.g. _POSIX_C_SOURCE says POSIX.1-2001
but _XOPEN_SOURCE says POSIX.1-2008).
Additionally, there is one special case in render/vulkan/vulkan.c.
That file needs major()/minor(), and these are system-specific.
On FreeBSD, _POSIX_C_SOURCE hides system-specific symbols so we need
to make sure it's not defined for this file. On Linux, we can
explicitly include <sys/sysmacros.h> and ensure that apart from
symbols defined there the file only uses POSIX toys.
We can just use a regular assignment instead. This is more
type-safe since there is no need to provide the struct size.
The remaining memcpy() calls perform array copies or copies from
void pointers (which may be unaligned).
