Use DWMEnableBlurBehindWindow instead of WS_EX_LAYERED

Affects per-pixel transparency

The current method renders to the screen by copying the GLES output to a
DIB for transparency using the CPU instead of rendering directly to the
window via the GPU. This is slower and also forces the window to be borderless
as WS_EX_LAYERED affects the non-client region as well.

This change uses DWMEnableBlurBehindWindow which allows using the standard
glClearColor() background alpha and is also performed through the GPU,
eliminating CPU bottlenecks

core/os/os.h

@@ -247,10 +247,6 @@ public:
 	virtual bool get_window_per_pixel_transparency_enabled() const { return false; }
 	virtual void set_window_per_pixel_transparency_enabled(bool p_enabled) {}
 
-	virtual uint8_t *get_layered_buffer_data() { return NULL; }
-	virtual Size2 get_layered_buffer_size() { return Size2(0, 0); }
-	virtual void swap_layered_buffer() {}
-
 	virtual void set_ime_active(const bool p_active) {}
 	virtual void set_ime_position(const Point2 &p_pos) {}
 	virtual Point2 get_ime_selection() const { return Point2(); }