Don't draw "blank" HEXTILE tiles with random data

Previously, if a HEXTILE tiles was received with a subencoding
of 0x00, it would draw a rectangle using data from the render
queue, which would result in random colored blocks when using
the HEXTILE encoding.  This is the result of a miscopy during
the refactoring.  It now has the correct functionality according
to the RFB protocol specification, which is to draw a rectangle
with the last set background color.

Closes #411