Normalize blend mode names.

test/pdfs/.gitignore

@@ -21,6 +21,7 @@
 !issue5874.pdf
 !issue5808.pdf
 !issue6204.pdf
+!issue6652.pdf
 !issue6782.pdf
 !issue6901.pdf
 !issue6961.pdf

test/pdfs/issue6652.pdf

@@ -0,0 +1,78 @@
+%PDF-1.7
+1 0 obj
+<<
+/Type /Catalog
+/Pages 2 0 R
+>>
+endobj
+2 0 obj
+<<
+/Type /Pages
+/MediaBox [ 0 0 100 100 ]
+/Count 1
+/Kids [ 3 0 R ]
+>>
+endobj
+3 0 obj
+<<
+/Type /Page
+/Parent 2 0 R
+/Contents 4 0 R
+/Resources <<
+/ExtGState <<
+/GS1 5 0 R
+/GS2 6 0 R
+>>
+>>
+>>
+endobj
+4 0 obj
+<<
+/Length 58
+>>
+stream
+/GS1 gs
+10 10 40 40 re
+f
+/GS2 gs
+1 1 1 rg
+20 20 40 40 re
+f
+endstream
+
+endobj
+5 0 obj
+<<
+/Type /ExtGState
+/BM /Multiply
+>>
+endobj
+6 0 obj
+<<
+/Type /ExtGState
+/BM /Compatible
+>>
+endobj
+xref
+0 1
+0000000000 65535 f
+1 1
+0000000009 00000 n
+2 1
+0000000058 00000 n
+3 1
+0000000143 00000 n
+4 1
+0000000262 00000 n
+5 1
+0000000371 00000 n
+6 1
+0000000423 00000 n
+trailer
+<<
+/Size 6
+/Root 1 0 R
+>>
+startxref
+477
+%%EOF

test/test_manifest.json

@@ -1899,6 +1899,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue6652",
+       "file": "pdfs/issue6652.pdf",
+       "md5": "1c8e2953f84623bc773eb720c87a9331",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue5801",
        "file": "pdfs/issue5801.pdf",
        "md5": "e9548650ad40e13e00d2a486bbc2bb1b",