Upgrade OpenGL backend to ES 3.0 (#995)

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

shaders/fill_outline.vertex.glsl

@@ -1,9 +1,9 @@
-attribute vec2 a_pos;
+layout (location = 0) in vec2 a_pos;
 
 uniform mat4 u_matrix;
 uniform vec2 u_world;
 
-varying vec2 v_pos;
+out vec2 v_pos;
 
 #pragma mapbox: define highp vec4 outline_color
 #pragma mapbox: define lowp float opacity