Ask the subclass to create a buffer with offset and size. When the
subclass returns GST_FLOW_OK, it MUST return a buffer of the requested size
unless fewer bytes are available because an EOS condition is near. No
buffer should be returned when the return value is different from
GST_FLOW_OK. A return value of GST_FLOW_EOS signifies that the end of
stream is reached. The default implementation will call
#GstBaseSrcClass::alloc and then call #GstBaseSrcClass::fill.
Ask the subclass to create a buffer with offset and size. When the subclass returns GST_FLOW_OK, it MUST return a buffer of the requested size unless fewer bytes are available because an EOS condition is near. No buffer should be returned when the return value is different from GST_FLOW_OK. A return value of GST_FLOW_EOS signifies that the end of stream is reached. The default implementation will call #GstBaseSrcClass::alloc and then call #GstBaseSrcClass::fill.