implemented DepthPacketProcessor using OpenGL shaders

This PR provides an accelerated implementation of the DepthPacketProcessor based on OpenGL shaders. It has the same functionality as the CpuDepthPacketProcessor. It introduces GLFW and GLEW as dependencies to handle OpenGL context creation and extension loading. 

TODOs until it can be merged:
 - [ ] move all constants in the GLSL shaders into a uniform struct
 - [ ] embed shaders into binary
 - [ ] extend test to ensure that output is 100% equivalent to CpuDepthPacketProcessor