WS_EX_TRANSPARENT 的真正含义是 ：
// If a WS_EX_TRANSPARENT window needs to be painted,
// and it has any non-WS_EX_TRANSPARENT windows siblings
// (which belong to the same process) which also need to be painted,
// then the window manager will paint the non-WS_EX_TRANSPARENT windows first.
// That "lets the other windows paint first" behavior is
// what gave the style its name, because if you let the other windows paint first,
// and then you paint on top of it, then the parts you don't actually paint
// will appear to be transparent because the other windows' pixels will show through.