Maxsize is initialized once and should never change. Allocating data
should have no impact on the selected max size for this memory. This
causing memory map failure as the maxsize would become smaller then
size. This happened when using direct rendering in avviddec on GL that
does not support PBO transfer.
https://bugzilla.gnome.org/show_bug.cgi?id=763045
e.g when wrapping a data pointer we don't want to map/unmap off the end of
pointer with the alignment bytes.
Instead track that information separately as maxsize is used for mapping by
GstMemory and thus represents a size without any alignment padding bytes.
This is made possible by a subclassable GstGLAllocationParams that holds
the allocation parameters
Every allocation would now go through gst_gl_base_memory_alloc with the
allocation parameters now being specified in a single struct to allow
extension by different allocators.
It handles the following
- GstAllocationParams -> gst_memory_init transformation
- Makes sure that map/unmap/create/destroy happen on the GL thread with
a GL context current.
- Holds a possible sysmem accessible data pointer with alignment.
- Holds the need upload/download transfer state
