Graph coloring

[erincatto]

Use graph coloring to multithread the active simulation. Islands are not used in the solver, but are still used for sleep.

Graph coloring results in many small jobs, so I implemented fine grained task coordination similar to bepu physics.

Implemented SIMD constraint solver making use of graph colors. Each color has fully independent constraints that may be solved simultaneously.

Performance using AVX SIMD was measured to be significantly faster than the island solver for large islands and many smaller islands. Scenarios with many large dynamic bodies interacting with many much smaller dynamic bodies may perform better with an island solver due to color overflow.

Tasks left to do before merging this PR.

• split islands
• overflow color
• time of impact
• fix crash with mouse joint on sleeping island
• fix build
• restitution