Geometric (GPU-less) picking

Redesign picking to avoid using the GPU. Previously picking involved
several GPU operations per mouse movement, but now the GPU/GL isn't
used at all.

By avoiding OpenGL and the graphics driver we also reduce CPU usage.
Despite reimplementing the logic on the CPU, it still takes less CPU
time than going through GL did.

Moving around a standard 125Hz mouse on an Intel i7-7700:
  before: 10% CPU, 1% GPU
  after:   6% CPU, 0% GPU
which is a 40% relative reduction in CPU usage according to 'top'.
Although according to Google Profiler the improvement can be between
35% and 64% (typically around 50%) relative reduction in CPU usage when
moving the mouse.

This new approach also dramatically reduces the number of picking paint
cycles required for cursor movement since the pickability of the entire
screen is calculated and cached. The cache is only invalidated when the
screen contents change so for typical desktop usage where the screen is
mostly idle, cursor movement doesn't incur anywhere near as many paint
cycles as it used to.

Closes: GNOME/mutter#154
Helps significantly with: GNOME/mutter#283
3 jobs for !189 with geometric-picking in 4 minutes and 53 seconds
detached
Status Job ID Name Coverage
  Review
passed #244007
check-commit-log

00:00:36

 
  Build
passed #244008
build-mutter

00:01:24

 
  Test
passed #244009
test-mutter

00:02:52