canvas: avoid emission of "draw" signal when the size is zero
canvas: avoid emission of "draw" signal when the size is zero

This also avoids the warning

  Cogl-WARNING **: ./cogl-buffer.c:215: GL error (1285): Out of memory

generated by cogl_buffer_map when the CoglBuffer has zero length.

djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded