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 testin...