Correct spelling of "tessellator" throughout code

Based on patch suggested by Homer Hsing

Bugzilla:  https://bugs.freedesktop.org/show_bug.cgi?id=50411

src/cairo-cogl-surface.c

@@ -143,7 +143,7 @@ typedef struct _cairo_cogl_path_fill_meta {
      * and translations but not for different scales.
      *
      * one idea is to track the diagonal lenghts of a unit rectangle
-     * transformed through the original ctm use to tesselate the geometry
+     * transformed through the original ctm use to tessellate the geometry
      * so we can check what the lengths are for any new ctm to know if
      * this geometry is compatible.
      */
@@ -167,7 +167,7 @@ typedef struct _cairo_cogl_path_stroke_meta {
      * and translations but not for different scales.
      *
      * one idea is to track the diagonal lenghts of a unit rectangle
-     * transformed through the original ctm use to tesselate the geometry
+     * transformed through the original ctm use to tessellate the geometry
      * so we can check what the lengths are for any new ctm to know if
      * this geometry is compatible.
      */
@@ -805,7 +805,7 @@ _cairo_cogl_journal_flush (cairo_cogl_surface_t *surface)
 		_cairo_path_fixed_approximate_clip_extents (&path->path, &extents);
 
 		/* TODO - maintain a fifo of the last 10 used clips with cached
-		 * primitives to see if we can avoid tesselating the path and
+		 * primitives to see if we can avoid tessellating the path and
 		 * uploading the vertices...
 		 */
 #if 0

src/cairo-fixed-private.h

@@ -46,7 +46,7 @@
 
 #if (CAIRO_FIXED_BITS != 32)
 # error CAIRO_FIXED_BITS must be 32, and the type must be a 32-bit type.
-# error To remove this limitation, you will have to fix the tesselator.
+# error To remove this limitation, you will have to fix the tessellator.
 #endif
 
 #define CAIRO_FIXED_ONE        ((cairo_fixed_t)(1 << CAIRO_FIXED_FRAC_BITS))

src/cairo-fixed-type-private.h

@@ -50,7 +50,7 @@ typedef cairo_int128_t	cairo_fixed_64_64_t;
 typedef cairo_int128_t	cairo_fixed_96_32_t;
 
 /* Eventually, we should allow changing this, but I think
- * there are some assumptions in the tesselator about the
+ * there are some assumptions in the tessellator about the
  * size of a fixed type.  For now, it must be 32.
  */
 #define CAIRO_FIXED_BITS	32