Fix forward and back animation issue

[Mikalai]

Description

There is an issue with this code

time = time_within_period(time, 2.0 * _maxTime);
if (time > _maxTime) time = 2.0 * _maxTime - time;

time value should run though the range of [0, 2 * maxTime] but as soon as it reaches maxTime it will stop grow and never reach 2.0 * _maxTime thus backward part won't work. I've added additional variable samplerTime which stores time for sampler while time variable will be able to move forward in the range [0, 2 * maxTime].

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Create simple forward and backward animation, start it and check incoming values to the sampler

auto animation = vsg::Animation::create();
animation->mode = vsg::Animation::FORWARD_AND_BACK;

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code