Spelling corrections: s/it's/its/

As a fun itch to scratch, I've been fixing incorrect uses of the
contraction "it's" in comments within the mozilla source tree (tracked
in https://bugzilla.mozilla.org/show_bug.cgi?id=458167 ), and I ran
across 6 instances of this typo in mozilla's snapshot of cairo.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-ft-font.c

@@ -2706,7 +2706,7 @@ cairo_ft_font_face_create_for_ft_face (FT_Face         face,
  * threaded application, because freetype's design makes it unsafe to
  * call freetype functions simultaneously from multiple threads, (even
  * if using distinct FT_Face objects). Because of this, application
- * code that acquires an FT_Face object with this call must add it's
+ * code that acquires an FT_Face object with this call must add its
  * own locking to protect any use of that object, (and which also must
  * protect any other calls into cairo as almost any cairo function
  * might result in a call into the freetype library).

src/cairo-matrix.c

@@ -513,7 +513,7 @@ _cairo_matrix_compute_adjoint (cairo_matrix_t *matrix)
  * cairo_matrix_invert:
  * @matrix: a #cairo_matrix_t
  *
- * Changes @matrix to be the inverse of it's original value. Not
+ * Changes @matrix to be the inverse of its original value. Not
  * all transformation matrices have inverses; if the matrix
  * collapses points together (it is <firstterm>degenerate</firstterm>),
  * then it has no inverse and this function will fail.

src/cairo-mutex-impl-private.h

@@ -53,7 +53,7 @@
 
 /* A fully qualified no-operation statement */
 #define CAIRO_MUTEX_IMPL_NOOP	do {/*no-op*/} while (0)
-/* And one that evaluates it's argument once */
+/* And one that evaluates its argument once */
 #define CAIRO_MUTEX_IMPL_NOOP1(expr)        do { (void)(expr); } while (0)
 /* Note: 'if (expr) {}' is an alternative to '(void)(expr);' that will 'use' the
  * result of __attribute__((warn_used_result)) functions. */

src/cairo-pdf-operators.c

@@ -120,7 +120,7 @@ _cairo_pdf_operators_enable_actual_text (cairo_pdf_operators_t *pdf_operators,
  * pdf_operators may leave the emitted PDF for some operations
  * unfinished in case subsequent operations can be merged. This
  * function will finish off any incomplete operation so the stream
- * will be in a state where the surface may emit it's own PDF
+ * will be in a state where the surface may emit its own PDF
  * operations (eg changing patterns).
  *
  */

src/cairo-surface-fallback.c

@@ -633,7 +633,7 @@ _clip_and_composite_trapezoids (const cairo_pattern_t *src,
              * If we have a clip surface, we set it as the mask; this only works
              * for bounded operators other than SOURCE; for unbounded operators,
              * clip and mask cannot be interchanged. For SOURCE, the operator
-             * as implemented by the backends is different in it's handling
+             * as implemented by the backends is different in its handling
              * of the mask then what we want.
              *
              * CAIRO_INT_STATUS_UNSUPPORTED will be returned if the region has