implement brush outline

src/app_shaders.cpp

@@ -161,7 +161,15 @@ void App::initShaders()
         "out mediump vec4 frag;\n"
         "void main() {\n"
         "   mediump float stroke = 1.0 - texture(tex, uv).r;\n"
-        "   frag = vec4(col.rgb, stroke * alpha);\n"
+        "   int zero_count = 0;\n"
+        "   for (int x = -1; x <= 1; x++){\n"
+        "       for (int y = -1; y <= 1; y++){\n"
+        "           if (textureOffset(tex, uv, ivec2(x, y)).r == 1.0) \n"
+        "               zero_count++;\n"
+        "       }\n"
+        "   }\n"
+        "   float edge = (zero_count > 1 && zero_count < 9) ? 0.75 : 0.0;\n"
+        "   frag = vec4(col.rgb, edge);\n"
         "}\n";
     // TEXTURE COMP ERASE
     static const char* shader_comp_erase_f =