Enforce line width to be at least 1px after applied transform

* add a comment to explain how minimal linewidth is computed.
 * when context.linewidth < 1 after transform, firefox and chrome
   don't render in the same way (issue #12810).
 * set lineWidth to 1 after transform and before stroking
   - aims fix issue #12295
   - a pixel can be transformed into a rectangle with both heights < 1.
     A single rescale leads to a rectangle with dim equals to 1 and
     the other to something greater than 1.
 * change the way to render rectangle with null dimensions:
   - right now we rely on the lineWidth set before "re" but
     it can be set after "re" and before "S" and in this case the rendering
     will be wrong.
   - render such rectangles as a single line.

test/pdfs/.gitignore

@@ -114,6 +114,7 @@
 !bug1050040.pdf
 !bug1200096.pdf
 !bug1068432.pdf
+!issue12295.pdf
 !bug1146106.pdf
 !bug1245391_reduced.pdf
 !bug1252420.pdf
@@ -277,6 +278,7 @@
 !gradientfill.pdf
 !bug903856.pdf
 !bug850854.pdf
+!issue12810.pdf
 !bug866395.pdf
 !issue12010_reduced.pdf
 !issue11718_reduced.pdf
@@ -313,6 +315,7 @@
 !issue3371.pdf
 !issue2956.pdf
 !issue2537r.pdf
+!issue12810.pdf
 !issue269_1.pdf
 !bug946506.pdf
 !issue3885.pdf

test/test_manifest.json

@@ -1190,6 +1190,12 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue12295",
+       "file": "pdfs/issue12295.pdf",
+       "md5": "c534f74866ba8ada56010d19b57231ec",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "bug1245391-text",
        "file": "pdfs/bug1245391_reduced.pdf",
        "md5": "6c946045ee0f2f663f269717c0f1614a",
@@ -2329,6 +2335,12 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue12810",
+       "file": "pdfs/issue12810.pdf",
+       "md5": "585e19781308603dd706f941b1ace774",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "pr4606",
        "file": "pdfs/pr4606.pdf",
        "md5": "6574fde2314648600056bd0e229df98c",
@@ -4324,6 +4336,12 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue12810",
+       "file": "pdfs/issue12810.pdf",
+       "md5": "585e19781308603dd706f941b1ace774",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "issue2956",
       "file": "pdfs/issue2956.pdf",
       "md5": "d8f68cbbb4bf54cde9f7f878acb6d7cd",