pdfium: use the variables rather than literal constants

[mara004]

Description

Avoid literal constants, and use the variables from the bindings instead.
Also, avoid a needless call of bitmap.get_info() (will disappear in v5). Instead, take the .mode from the bitmap object directly.
Disclaimer: untested, patch created through GH's web interface.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[jdye64]

/ok to test