Reformat Framebuffer::readPixels and its overriding methods
to the following method signature

angle::Result readPixels(const Context *context,
                         const Rectangle &area,
                         GLenum format,
                         GLenum type,
                         const PixelPackState &pack,
                         Buffer *packBuffer
                         void *pixels);

This will allow capture replay tool to use its own PixelPackState
to read pixels from framebuffer without having to set the global states

Bug: angleproject:4787
Change-Id: Idc64179d8e8f6b5163ef0747f239cd5172a2491b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2267417
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>