Added a workaround for OpenGL bended lines

There are two reasons why the lines might become bended:
1) swapBuffers() call waits for the display VSync for about 6 ms, all the
   events seem to be dropped during this time.
2) When painting very *huge* circles with very small brush
   (circles ~ 3000px in diameter, brush ~5px) some another factor
   appears. I cannot tell for sure, but this is, probably, texture loading.

This patch fixes only the first (!) reason of line bending.
First, it asks Qt to use single buffered contexts. Then, if a single
buffered context is not available, it asks openGL to disable VSync.

CCBUG:300339