Implement GPollableInputStream in GMemoryInputStream and
GConverterInputStream, and likewise implement GPollableOutputStream in
the corresponding output streams.
https://bugzilla.gnome.org/show_bug.cgi?id=673997
==8221== 1,047 (672 direct, 375 indirect) bytes in 28 blocks are definitely lost in loss record 589 of 603
==8221== at 0x4005BDC: malloc (vg_replace_malloc.c:195)
==8221== by 0x4057094: g_malloc (gmem.c:134)
==8221== by 0x406F2D6: g_slice_alloc (gslice.c:836)
==8221== by 0x406F31B: g_slice_alloc0 (gslice.c:848)
==8221== by 0x403A8A6: g_error_new_literal (gerror.c:117)
==8221== by 0x403AC31: g_set_error_literal (gerror.c:314)
==8221== by 0x80499DC: g_compressor_converter_convert (converter-stream.c:267)
==8221== by 0x417BF67: g_converter_convert (gconverter.c:174)
==8221== by 0x417D7F0: g_converter_output_stream_write (gconverteroutputstream.c:428)
==8221== by 0x41B57DF: g_output_stream_write (goutputstream.c:216)
==8221== by 0x804A367: test_compressor (converter-stream.c:456)
Bug #628309.
The name buffer_availabile was kinda confusing, so its been renamed
to buffer_data_size() to match buffer_data().
Also I added a comment to buffer_ensure_space because its behaviour
wasn't obvious.
When the converter fills the whole buffer without reading all input,
we need to enlarge the buffer. Otherwise we get an assertion failure
for `outbuf_size > 0' in g_converter_convert.
https://bugzilla.gnome.org/show_bug.cgi?id=619945
