Avoid converting screenshot readback to ARGB32

OpenGL writes pixels as GL_RGBA, but the buffer is declared as QImage::Format_ARGB32_Premultiplied, so KWin manually iterates over the entire image and swaps the red and blue channels, with separate byte-order handling.

The conversion to QImage::Format_ARGB32_Premultiplied is not needed at all. Modern Qt supports the byte-ordered QImage::Format_RGBA8888_Premultiplied, so after reading the data from OpenGL, KWin now only flips the image vertically (and this could potentially be improved in the future as well).