Remove a duplicated reference test (PR 12812 follow-up)

- Remove a *duplicated* reference test, see "issue12810", from the manifest.

 - Improve the spelling in a couple of comments in `src/core/canvas.js`, most notable of the word "parallelogram".

 - Update a comment, also in `src/core/canvas.js`, to actually agree with the value used to reduce confusion when reading the code.

test/pdfs/.gitignore

@@ -316,7 +316,6 @@
 !issue3371.pdf
 !issue2956.pdf
 !issue2537r.pdf
-!issue12810.pdf
 !issue269_1.pdf
 !bug946506.pdf
 !issue3885.pdf

test/test_manifest.json

@@ -4342,12 +4342,6 @@
        "rounds": 1,
        "type": "eq"
     },
-    {  "id": "issue12810",
-       "file": "pdfs/issue12810.pdf",
-       "md5": "585e19781308603dd706f941b1ace774",
-       "rounds": 1,
-       "type": "eq"
-    },
     {  "id": "issue2956",
       "file": "pdfs/issue2956.pdf",
       "md5": "d8f68cbbb4bf54cde9f7f878acb6d7cd",