Support shadows

[Adnn]

• [x] Use shadow sampler
• [x] Add bias to eliminate shadow acne
• [x] Get proper phong lighting of the ground, and shadow on the avocados
  • [x] Generate normals
• [x] Reduce the number of programs (potentially reuse shader sections with defines
  • [x] Texture/Plain and Shadow/NoShadow
• [x] Only build shadow "pipelines" resource once (instead of each frame)
• [ ] Consolidate the "recompile" shaders to work in all modes.
• [x] Redesign the Render API (should allow to set uniforms / textures at different levels)