Rewrite Block Analyzer to use pure OpenGL instead of QGLPaintEngine2.

This rewrite should fix a number of issues that users had, especially
with Intel drivers. The issues, including crashes, are all due to driver
bugs, but essentially they were triggered by Qt using some uncommon
features for texture drawing (stencil operations, etc).

This commit should be backported to 2.8.1.

CCMAIL: amarok-devel@kde.org
CCBUG: 323635
BACKPORT