Fix loading of PDF files with invalid or missing Type3 characters (issue 5039)
Optimize Ref_toString().

I have a large PDF where this function is called 1.6 million times
during loading. Minimizing the string concatenations reduces the
cumulative allocations done by Firefox within this function from 113 MB
to 48 MB.
Initialize PDFView.url = ''

The absence of this property caused the following error in PDFView.download:

   Cannot read property 'split' of undefined

This property was unset when the PDF was loaded through a typed array instead
of a URL. With the fix, the file name will default to "document.pdf", as
defined by getPDFFileNameFromURL in web/ui_utils.js
Use Array.join to build up strings in readPostScriptTable().

This avoids about 5 MiB of string allocations on one test case.
Merge pull request #5078 from nnethercote/Ref-toString

Optimize Ref_toString().
Merge pull request #5083 from Rob--W/pdfview-url-property

Initialize PDFView.url = ''
Added clip operator
Added fill, stroke, line-join & font attr
Misc variable initialization
Fixed setGState, viewport transform
Added endPath, endText. fixes extra <g> elements
Merge pull request #5034 from pramodhkp/fixupgstate2

[SVG] Adds clip operator, fixes setGState, fill, stroke operators
Merge pull request #5074 from nnethercote/readPostScriptTable-join

Use Array.join to build up strings in readPostScriptTable().
Merge pull request #5077 from Snuffleupagus/issue-5039

Fix loading of PDF files with invalid or missing Type3 characters (issue 5039)