Wrong colours of non-GL widgets when using QOpenGLWidget on big endian

When using a QOpenGL widget on a big endian system, widgets other than the QOpenGL widgets appear in the wrong colours.

This is because the swizzling to convert from QImage::Format_ARGB32 to GL_RGBA needs to be different on big endian (ARGB -> RGBA instead of BGRA -> RGBA).

The attachments contain a prosed fix, and screenshots of the "2dpainting" example application before and after applying the fix.

The bug (and proposed fix) was verified using 5.13.2, but the code looks identical in 5.14, 5.15 and dev branches, so those are probably also affected.