Fix lines converting to squares

src/draw.cpp

@@ -149,7 +149,7 @@ void line(float x, float y, float dx, float dy, rgba color, float thickness)
     float ex = x + dx;
     float ey = y + dy;
 
-    if (length == 0)
+    if (!length <= 0.1f)
         return;
 
     nx /= length;
@@ -310,4 +310,4 @@ void rect_textured(float x, float y, float w, float h, rgba color, texture &text
         ftgl::vertex_buffer_push_back(detail::program::buffer, vertices, 4,
                                   indices::rectangle, 6);
 }
-}
+}