a0.10_9 Misc changes and big shader optimisation & cleanup

+ added gradient check before light calculation;
+ circles now indicate where lights are located when in lighting mode;
+ added controls section to README;
+ added tool icon in lower-left for better usability;
* renamed light.size to light.radius for clarity;
* organised lighting.frag into functions;
* changed to use 'out vec4 fragColor' rather than deprecated gl_FragColor;
- removed uApple & uSmoothShadows shader uniforms (no more manual toggling of soft/hard shadows);