Upgrade OpenGL backend to ES 3.0 (#995)

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

shaders/symbol_icon.fragment.glsl

@@ -1,7 +1,7 @@
 uniform sampler2D u_texture;
 
-varying vec2 v_tex;
-varying float v_fade_opacity;
+in vec2 v_tex;
+in float v_fade_opacity;
 
 #pragma mapbox: define lowp float opacity
 
@@ -9,9 +9,9 @@ void main() {
     #pragma mapbox: initialize lowp float opacity
 
     lowp float alpha = opacity * v_fade_opacity;
-    gl_FragColor = texture2D(u_texture, v_tex) * alpha;
+    fragColor = texture(u_texture, v_tex) * alpha;
 
 #ifdef OVERDRAW_INSPECTOR
-    gl_FragColor = vec4(1.0);
+    fragColor = vec4(1.0);
 #endif
 }