Add deterministic noise to building heights

- Add deterministic noise to buildings heights
  - To do this, need to assign a unique x and y ID for each building.
    These IDs serve as the initial seed for the random function. Then
    multiply `zFac` by a random float in [0.25, 1.25].
- Remove duplicate code for helper functions in the vertex shader
  (No need for entirely redefined functions for the two cases, can
  share most of the code)