EGL/GLES Compatibility bug in grid.shader

[jwalt]

Hi!

Using ARM Mali drivers, Cura's gid.shader does not compile. There are two issues with it.

• One is the same as Ultimaker/Uranium#117 (and the suggested fix works fine)
• The other one is that function fwidth is missing. The driver does support fwidth, it just isn't available by default in EGL/GLESv2 and must be explicitly requested

A shader preamble like this works perfectly:

fragment =
    #extension GL_OES_standard_derivatives : enable
    #ifdef GL_ES
        #ifdef GL_FRAGMENT_PRECISION_HIGH
            precision highp float;
        #else
            precision mediump float;
        #endif // GL_FRAGMENT_PRECISION_HIGH
    #endif // GL_ES

[thopiekar]

Could you provide a PR for the second point?

Thanks!

[Ghostkeeper]

I'd rather put that extra line inside the #ifdef GL_ES, since you say that the disabled fwidth is only a problem in EGL/GLESv2. Maybe you could check if this still works?

fragment =
    #ifdef GL_ES
        #extension GL_OES_standard_derivatives : enable
        #ifdef GL_FRAGMENT_PRECISION_HIGH
            precision highp float;
        #else
            precision mediump float;
        #endif // GL_FRAGMENT_PRECISION_HIGH
    #endif // GL_ES

[jwalt]

@Ghostkeeper , yes that works. My PR contains that version.