Upgrade OpenGL backend to ES 3.0 (#995)

Co-authored-by: Tadej Novak <tadej@tano.si>

shaders/fill_outline.fragment.glsl

@@ -1,4 +1,4 @@
-varying vec2 v_pos;
+in vec2 v_pos;
 
 #pragma mapbox: define highp vec4 outline_color
 #pragma mapbox: define lowp float opacity
@@ -9,9 +9,9 @@ void main() {
 
     float dist = length(v_pos - gl_FragCoord.xy);
     float alpha = 1.0 - smoothstep(0.0, 1.0, dist);
-    gl_FragColor = outline_color * (alpha * opacity);
+    fragColor = outline_color * (alpha * opacity);
 
 #ifdef OVERDRAW_INSPECTOR
-    gl_FragColor = vec4(1.0);
+    fragColor = vec4(1.0);
 #endif
 }