glimagesink: last-sample property useless for planar YUV formats

Submitted by Sebastian Dröge @slomo

Link to original bug (#734223)

Description

W/GStreamer+default(10996): 0:00:05.788143615 0x608b7af0 gstglutils.c:522:gst_gl_context_set_error Cannot download GL luminance/luminance alpha textures

This makes the convert-sample and last-sample properties on playbin useless with planar YUV formats. Can we somehow keep around the original data until it is changed in GL and use that for mapping instead of downloading from GL?