Clarify Props.SetFramebufferSize more.

[slimsag]

Props.SetFramebufferSize mentions a vague window owner:

// SetFramebufferSize sets the size of the framebuffer in pixels. Each value is
// clamped to at least a value of 1.
//
// Only the window owner should ever set the framebuffer size.

It should instead read:

// SetFramebufferSize sets the size of the framebuffer in pixels. Each value is
// clamped to at least a value of 1.
//
// Only the Window implementation should set the framebuffer size: clients who are just
// utilizing the existing implementations defined in this package should not invoke this
// method.