ssbo breakthrough: 5x gain by shrinking the data

- pack entity data (x, y, color) into 12-byte struct
- upload via shader storage buffer object (SSBO)
- ~700k entities at 60fps (i5-6500T / HD 530)
- ~950k entities at ~57fps
- 5x improvement over previous best
- 140x total from baseline
- why it works:
  - 12 bytes vs 64 bytes (matrices) = 5.3x less bandwidth
  - 12 bytes vs 80 bytes (rlgl vertices) = 6.7x less bandwidth
  - no CPU-side matrix calculations
  - GPU does NDC conversion and color unpacking
- custom vertex/fragment shaders
- single rlDrawVertexArrayInstanced() call for all entities
- shaders embedded at build time
- removed FPS cap, added optional vsync arg