ioemu rendering fixes

ioemu rendering fixes

- no need to call the colourdepth callback in text mode: the buffer
  cannot be shared anyway;

- line size changes are currently undetected: since we added a
  linesize parameter to the resize callback, we also need to detect
  line size changes and call dpy_resize accordingly;

- 8 bit colour depth with the shared framebuffer is broken: in order
  to fix it I disabled the shared buffer in vnc for the 8bit colour
  depth case (it has to be done in software anyway..) and implemented
  paletted colours in both opengl and sdl;

- opengl rendering is broken when there is padding in the framebuffer
  lines: removing unnecessary GL_UNPACK_ALIGNMENT settings so that the
  GL_UNPACK_ROW_LENGTH parameter can work properly.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>