Fix memory leak in Context::pop_group() due to double reference count

[hana-kami]

Bug Description

Context::pop_group() causes a memory leak because it passes
has_reference=false to get_pattern_wrapper(), which unconditionally
calls cairo_pattern_reference() to take an extra reference. However,
cairo_pop_group() already returns a pattern with an initial refcount of
1 — the caller owns that reference. This results in the refcount being
inflated by 1 and never reaching zero.

Reference Count Trace (before fix)

Step	Operation	refcount
1	cairo_pop_group() returns pattern (caller owns it)	1
2	get_pattern_wrapper(pattern, false) → calls cairo_pattern_reference()	2
3	cairo_set_source() inside set_source() adds a reference	3
4	Temporary RefPtr<Pattern> is destroyed → cairo_pattern_destroy()	2
5	Context replaces source (e.g. on restore()) → cairo_pattern_destroy()	1
6	No pointer to the pattern remains → definite leak	1 (leaked)

This was confirmed with Valgrind:

21,960 bytes in 9 blocks are definitely lost
  at cairo_pattern_create_for_surface (cairo-pattern.c:771)
  by _cairo_default_context_pop_group (cairo-default-context.c:238)
  by cairo_pop_group (cairo.c:768)
  by Cairo::Context::pop_group() (context.cc:885)

Root Cause

get_pattern_wrapper() was designed for get_source(), where
cairo_get_source() returns a borrowed reference (the context still
owns it), so the wrapper must call cairo_pattern_reference() to obtain
its own reference (has_reference=false is correct there).

But cairo_pop_group() returns a new reference (the caller is
responsible for calling cairo_pattern_destroy()), so the wrapper must
NOT call cairo_pattern_reference() again.

Fix

Add a has_reference parameter (defaulting to false) to
get_pattern_wrapper() so callers can specify whether they already own
the reference. Update pop_group() to pass true.

• get_source() continues to use the default false (borrowed reference → correct).
• pop_group() now passes true (owned reference → no extra ref).

Testing

Verified with Valgrind that the "definitely lost" leak no longer appears
after applying this fix.