Move compositor frame ownership into the view renderers (#189162)

Make FlCompositorSoftware and FlCompositorOpenGL stateless so they only
composite layers into a caller-provided target. The view renderers now
own the Cairo surface / OpenGL framebuffer and CPU pixel buffers, size
them, and read frames back as required.

Add FlOpenGLFrame to encapsulate the OpenGL framebuffer and CPU pixel
copy managed by the OpenGL renderer, and move the shared window-resize
logic into a common fl_view_renderer_resize_to_frame method on
FlViewRenderer. Drop the now unused return value from
fl_compositor_opengl_composite_layers. Update the tests and compositor
class descriptions to match.

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Co-authored-by: Matthew Kosarek <matt.kosarek@canonical.com>