Font renderer - get int8 instead of uint8 in composite glyphes (bug 1749563)

- it aims to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1749563;
 - use some helper functions to get (u|i)int** values in buffer: it helps to have a clearer code;
 - in composite glyphes the translations values with a transformations are signed so consequently get some int8 instead of uint8;
 - add few TODOs.

test/pdfs/bug1749563.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=9258518

test/test_manifest.json

@@ -6215,5 +6215,14 @@
        "rounds": 1,
        "type": "eq",
        "annotations": true
-    }
+    },
+    { "id": "bug1749563",
+      "file": "pdfs/bug1749563.pdf",
+      "md5": "11294f6071a8dcc25b0e18953cee68fa",
+      "rounds": 1,
+      "link": true,
+      "firstPage": 1,
+      "lastPage": 1,
+      "type": "eq"
+   }
 ]