Upgrade OpenGL backend to ES 3.0 (#995)

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

shaders/background_pattern.vertex.glsl

@@ -7,10 +7,9 @@ uniform float u_scale_a;
 uniform float u_scale_b;
 uniform float u_tile_units_to_pixels;
 
-attribute vec2 a_pos;
-
-varying vec2 v_pos_a;
-varying vec2 v_pos_b;
+layout (location = 0) in vec2 a_pos;
+out vec2 v_pos_a;
+out vec2 v_pos_b;
 
 void main() {
     gl_Position = u_matrix * vec4(a_pos, 0, 1);