Some downstream user might want to use the very handy comp_buffer_init()
helper to create buffers, but the latter does not cover the case when
the buffer needs to be exportable for API interoperability.
Add a new comp_buffer_init_exportable() helper to cover that case, this
is done in a way that is not particularly invasive for existing users of
comp_buffer_init(), as all the logic about the exportability is handled
by the new function.
The size field in comp_buffer was never assigned, and also never used as
all comp_buffer users rely on the allocation_size field.
Use the size field to store the size originally requested when creating
the buffer which could be different from the value returned in the
allocation_size field.
Having both sizes available allows a user to check if allocation_size is
in fact different from the requested size.
This matches the OpenXR usage: the array is the plural of the element type,
and the count is the singular element type plus "count" (usually CountOutput
because of the two-call idiom)
Includes fixes to other code to match API changes.