Flycontrols gets stuck on yaw/pitch with mousedown.

[dcsturman]

• three version: 0.173.0
• @types/three version: 0.163.0
• three-stdlib version: 2.35.6

Problem description:

When using Flycontrols and specifically using mousedown for yaw and pitch, the mouse can get "stuck" so that when released, mouse movement still cuases yaw and pitch. Its then impossible to get out of this mode.

Relevant code:

Suggested solution:

It seems the fix is straightforward here. See:
#406