Custom shaders

This MR contains a custom mapper and improvements to the light class.

Edit: I removed normal mapping an tissue shader for now to make this merge request smaller. Plus, as we discuss shader work plans, the implementation of this may change.

Custom shaders (#108 (closed)):

• Implement metal shaders
• Implement linear color space in shader
• Implement tone mapping

Improved light class (#137 (closed)):

The light class has changed, and will undergo more changes:

• Light intensity has been added since there's currently no way to have a light brighter than (1, 1, 1)
• Lights will follow the inverse square law (physically accurate and cheaper to compute) rather than a quadratic attenuation function. It also makes more intuitive sense (only only parameter to control rather than 3).
• Support for up to 16 lights (VTK limit is 6, but the custom mapper can break free from this)
• Renaming of lights: lights should be renamed to something more consistent with computer graphics (scene lights -> point lights/spot lights, etc.)