Replace various FPDFText_* API usage inside pdfium_page.cc

Now that FPDFText_GetTextObject() exists, many existing FPDFText_* API
calls inside pdfium_page.cc can be replaced with an equivalent
FPDFTextObj_* API call, or FPDFFont_* API call:

FPDFText_GetFontSize()    -> FPDFTextObj_GetFontSize()
FPDFText_GetFontInfo()    -> FPDFFont_GetBaseFontName() and
                             FPDFFont_GetFlags()
FPDFText_GetFontWeight()  -> FPDFFont_GetWeight()
FPDFText_GetFillColor()   -> FPDFPageObj_GetFillColor()
FPDFText_GetStrokeColor() -> FPDFText_GetStrokeColor()

Make these conversions and add more error checking as needed, to clear
the way for removing these FPDFText_* APIs from PDFium.

As part of the conversions, many functions that take the pair
(FPDF_TEXTPAGE, int /*char_index*/) can be simplified to just take
FPDF_PAGEOBJECT. Do this recursively to simplify more related code.

Bug: 353746891, 353746892
Change-Id: I846093a8ee5979415eefa2121936c3ba81b08c49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5719478
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Alan Screen <awscreen@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1333795}