Fix shaders on iOS

[jsfdez]

QOpenGLShader::compile(Fragment): ERROR: 0:7: 'vec4' : declaration must include a precision qualifier for type
ERROR: 0:8: Use of undeclared identifier 'c'
ERROR: 0:8: Use of undeclared identifier 'c'

2020-03-26T18:27:56 (default.warning): *** Problematic Fragment shader source code ***
#ifdef GL_KHR_blend_equation_advanced
#extension GL_ARB_fragment_coord_conventions : enable
#extension GL_KHR_blend_equation_advanced : enable
#endif
#ifndef GL_FRAGMENT_PRECISION_HIGH
#define highp mediump
#endif
#line 1
uniform sampler2D tex;
uniform lowp float opacity;
varying highp vec2 uv;
varying highp vec4 color;
void main() {
    vec4 c = color * texture2D(tex, uv);
    gl_FragColor = vec4(c.rgb, c.a * opacity);
}

***