add an overdraw inspector

The overdraw inspector shows how many time each fragment has been
shaded. White fragments were shaded 8 or more times. Black fragments
were shaded 0 times. Lower is better.

`map.showOverdrawInspector = true` turns it on.

circle.fragment.glsl

@@ -10,4 +10,8 @@ varying lowp float v_antialiasblur;
 void main() {
     float t = smoothstep(1.0 - max(u_blur, v_antialiasblur), 1.0, length(v_extrude));
     gl_FragColor = v_color * (1.0 - t) * u_opacity;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

fill.fragment.glsl

@@ -5,4 +5,8 @@ uniform lowp float u_opacity;
 
 void main() {
     gl_FragColor = u_color * u_opacity;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

icon.fragment.glsl

@@ -10,4 +10,8 @@ varying vec2 v_fade_tex;
 void main() {
     lowp float alpha = texture2D(u_fadetexture, v_fade_tex).a * u_opacity;
     gl_FragColor = texture2D(u_texture, v_tex) * alpha;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

line.fragment.glsl

@@ -20,4 +20,8 @@ void main() {
     float alpha = clamp(min(dist - (u_linewidth.t - blur), u_linewidth.s - dist) / blur, 0.0, 1.0);
 
     gl_FragColor = u_color * (alpha * u_opacity);
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

linepattern.fragment.glsl

@@ -41,4 +41,8 @@ void main() {
     alpha *= u_opacity;
 
     gl_FragColor = color * alpha;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

linesdfpattern.fragment.glsl

@@ -29,4 +29,8 @@ void main() {
     alpha *= smoothstep(0.5 - u_sdfgamma, 0.5 + u_sdfgamma, sdfdist);
 
     gl_FragColor = u_color * (alpha * u_opacity);
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

outline.fragment.glsl

@@ -9,4 +9,8 @@ void main() {
     float dist = length(v_pos - gl_FragCoord.xy);
     float alpha = smoothstep(1.0, 0.0, dist);
     gl_FragColor = u_color * (alpha * u_opacity);
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

outlinepattern.fragment.glsl

@@ -29,4 +29,8 @@ void main() {
 
 
     gl_FragColor = mix(color1, color2, u_mix) * alpha * u_opacity;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

pattern.fragment.glsl

@@ -23,4 +23,8 @@ void main() {
     vec4 color2 = texture2D(u_image, pos2);
 
     gl_FragColor = mix(color1, color2, u_mix) * u_opacity;
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

raster.fragment.glsl

@@ -40,4 +40,8 @@ void main() {
     vec3 u_low_vec = vec3(u_brightness_high, u_brightness_high, u_brightness_high);
 
     gl_FragColor = vec4(mix(u_high_vec, u_low_vec, rgb), color.a);
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }

sdf.fragment.glsl

@@ -17,4 +17,8 @@ void main() {
     lowp float gamma = u_gamma * v_gamma_scale;
     lowp float alpha = smoothstep(u_buffer - gamma, u_buffer + gamma, dist) * fade_alpha;
     gl_FragColor = u_color * (alpha * u_opacity);
+
+#ifdef OVERDRAW_INSPECTOR
+    gl_FragColor = vec4(1.0);
+#endif
 }