refactor src/core/pdf_manager.js: rename pdfModel to pdfDocument

Rename pdfModel to pdfDocument to let the name closer resemble what the
variable actually holds
Add Test for PDFDocumentProxy_getPageIndex
Introduce paintSolidColorImageMask command to handle 1x1 solid image masks

See https://github.com/mozilla/pdf.js/issues/4436 for details
Added TC for issue 4436
Merge pull request #4481 from bthorben/refactor/pdf_manager.js

refactor src/core/pdf_manager.js: rename pdfModel to pdfDocument
Sync pt-BR translations with l10n-mozilla-aurora
Merge pull request #4488 from alexandrevicenzi/syncptBRtranslations

Sync pt-BR translations with l10n-mozilla-aurora
Merge pull request #4474 from chriskr/draw-image-subpixel-support

Introduce paintSolidColorImageMask command to handle 1x1 solid image