Improve PBR textures

• The LUT texture is now using GL_RG16 instead of GL_RG16F:
  • Allow usage of native unsigned short when downloading pixels instead of half-float
  • Makes more sense because the values are clamped between 0 and 1, so the quality is better using normalized values
• The prefilter specular texture is now optionally using GL_RGB32F instead of GL_RGB16F:
  • Allow usage of native float when downloading pixels instead of half-float
  • Support wider HDR values. Of course, the drawback is VRAM usage is doubled, but it's disabled by default.