GLES error on Raspberry Pi
Hi,

I have been trying to run picolove on the Raspberry Pi platform.
It's running raspbian and SDL2 is built with GLES2 support. Many SDL2 games work on it using the GLES2 renderer.
On the LÖVE side of things, the SuperToast demo works perfectly too.
Howeve, picolove gives me this:

```
pi@raspberrypi:~/pico $ love picolove/
Error: Cannot link shader program object:
ERROR:CUSTOM-14 (fragment shader, line 5) Support for indexing array/vector/matrix with a non-constant is not mandated in the fragment shader
stack traceback:
    [C]: in function 'newShader'
    main.lua:244: in function 'load'
    main.lua:659: in function <main.lua:647>
    [C]: in function 'xpcall'

```

Any idea on what could be wrong here?

thanks!
