Stop creating cairo contexts in haiku_begin_cr_clip

Stop creating cairo contexts in haiku_begin_cr_clip

* src/haikufont.c (haikufont_draw):
* src/ftcrfont.c (ftcrfont_draw): Stop holding the draw lock.
* src/haiku_support.cc (class EmacsView): New field
`cairo_context'.
(DetachCairoSurface): Destroy cairo_context.
(AttachCairoSurface): Create cairo_context from the surface.
(EmacsView_cairo_surface): Delete function.
(EmacsView_cairo_context): New function.
* src/haiku_support.h: Update prototypes.
* src/haikuterm.c (haiku_begin_cr_clip):
(haiku_end_cr_clip): Retrieve the existing cairo context instead
of creating a new one.
* src/haikuterm.h (FRAME_CR_CONTEXT): New macro.