Fix broken tests from changing default screen size

Commit 795fca23dc6cf124b2e6969ee42bd8e7c836d605 changed the default
size from 640 to 240.  This broke a couple tests which depended on
the default size being 640.  Those tests have now been fixed.
Fix Karma sinon-chai version issues

Previously, we were using the karma-sinon-chai package to
provide sinon-chai to karma.  This used an older version of
sinon-chai, and looks to be no longer maintained (it's been
a month since sinon-chai was updated).  A new package,
karma-sinon-chai-latest, is now used.  This package uses the
latest version of sinon-chai and sinon, just like karma-chai
uses the latest version of chai.
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
Merge "pull-request/412" (ramondeklein/noVNC)

Closes #412