Revert "device: no need for alpha"

This reverts commit 734473761497b7e59a0e26a849cb19d0ee75732c.

src/device/pipeline.cpp

@@ -445,7 +445,7 @@ void Pipeline::reset(u16 width, u16 height)
 			, height
 			, false
 			, 1
-			, bgfx::TextureFormat::RGB8
+			, bgfx::TextureFormat::RGBA8
 			, color_texture_flags
 			),
 		_depth_texture