Is it possible to create Line and Watermark annotation into pdf?

[Loc Zhang (上杉夏香)]

When I call FPDFPage_CreateAnnot to create line and watermark annotation, I get an error.

And I see FPDFAnnot_IsSupportedSubtype 

```c

FPDF_EXPORT FPDF_BOOL FPDF_CALLCONV

FPDFAnnot_IsSupportedSubtype(FPDF_ANNOTATION_SUBTYPE subtype) {

// The supported subtypes must also be communicated in the user doc.

switch (subtype) {

case FPDF_ANNOT_CIRCLE:

case FPDF_ANNOT_FILEATTACHMENT:

case FPDF_ANNOT_FREETEXT:

case FPDF_ANNOT_HIGHLIGHT:

case FPDF_ANNOT_INK:

case FPDF_ANNOT_LINK:

case FPDF_ANNOT_POPUP:

case FPDF_ANNOT_SQUARE:

case FPDF_ANNOT_SQUIGGLY:

case FPDF_ANNOT_STAMP:

case FPDF_ANNOT_STRIKEOUT:

case FPDF_ANNOT_TEXT:

case FPDF_ANNOT_UNDERLINE:

return true;

default:

return false;

}

}

```

There is no way use pdfium to create annotation like line?

[Lei Zhang]

Please add this as 2 separate feature requests on the bug tracker:
https://crbug.com/pdfium/new, one for each annotation sub-type.

Is it possible? Yes. But someone has to do the work and teach PDFium
how to generate the appropriate annotation dictionary fields for each
annotation sub-type.

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