render: Add focus indicator

src/backend/render/shaders/focus_indicator.frag

@@ -0,0 +1,33 @@
+precision mediump float;
+uniform float alpha;
+#if defined(DEBUG_FLAGS)
+uniform float tint;
+#endif
+uniform vec2 size;
+varying vec2 v_coords;
+
+uniform vec3 color;
+uniform float thickness;
+uniform float radius;
+
+float rounded_box(vec2 center, vec2 size, float radius) {
+    return length(max(abs(center) - size + radius, 0.0)) - radius;
+}
+
+void main() {
+    vec2 center = size / 2.0 - vec2(0.5);
+    vec2 location = v_coords * size;
+    vec4 mix_color;
+
+    float distance = rounded_box(location - center, size / 2.0 - vec2(thickness / 2.0), radius);
+    float smoothedAlpha = 1.0 - smoothstep(0.0, 2.0, abs(distance) - (thickness / 2.0));
+    
+    mix_color = mix(vec4(0.0, 0.0, 0.0, 0.0), vec4(color, alpha), smoothedAlpha);
+    
+#if defined(DEBUG_FLAGS)
+    if (tint == 1.0)
+        mix_color = vec4(0.0, 0.3, 0.0, 0.2) + mix_color * 0.8;
+#endif
+
+    gl_FragColor = mix_color;
+}