Use bilinear interpolation to compute texture coords of subquads
On Wayland, a surface must be displayed the same way no matter how the attached buffer is transformed. In order to guarantee that, we build the surface-to-buffer matrix, which is used to compute the texture coords.
The surface-to-buffer matrix represents an affine transformation. Thus, performing linear interpolation between texture coordinates won't end up in corrupted rendered results. This is the main assumption that we make during generation of contents window quads. After creating a sub-quad, the new quad's texture coordinates are computed by interpolating between the source quad's texture coords.
However, WindowQuad::makeSubQuad() makes a concrete assumption about the order of texture coords, which might be false if the attached wayland buffer is rotated 90 or 270 degrees.
This issue went unnoticed after merging the viewporter patches because the developer who was working on it had been using primarily nested kwin_wayland for testing purposes. And it appears like kwin schedules full screen repaints even though it supports buffer age. It still needs some investigation why that happens.