FFI: Add MuPDF fz_is_document_reflowable (#960)

This will be queried in front to dynamically set font size.

Cf. <koreader/koreader#4368>.

ffi-cdecl/mupdf_decl.c

@@ -88,6 +88,7 @@ cdecl_struct(fz_page_s)
 
 cdecl_func(mupdf_open_document)
 cdecl_func(mupdf_open_document_with_stream)
+cdecl_func(fz_is_document_reflowable)
 cdecl_func(fz_needs_password)
 cdecl_func(fz_authenticate_password)
 cdecl_func(fz_drop_document) // NOTE: libk2pdfopt uses old fz_free_document symbol

ffi/mupdf.lua

@@ -171,6 +171,12 @@ function document_mt.__index:getPages()
     return pages
 end
 
+function document_mt.__index:isDocumentReflowable(width, height, em)
+    if self.is_reflowable then return self.is_reflowable end
+    self.is_reflowable = M.fz_is_document_reflowable(context(), self.doc) == 1
+    return self.is_reflowable
+end
+
 function document_mt.__index:layoutDocument(width, height, em)
     -- Reset the cache.
     self.number_of_pages = nil

ffi/mupdf_h.lua

@@ -182,6 +182,7 @@ struct fz_page_s {
 };
 fz_document *mupdf_open_document(fz_context *, const char *);
 fz_document *mupdf_open_document_with_stream(fz_context *, const char *, struct fz_stream_s *);
+int fz_is_document_reflowable(fz_context *, fz_document *);
 int fz_needs_password(fz_context *, fz_document *);
 int fz_authenticate_password(fz_context *, fz_document *, const char *);
 void fz_drop_document(fz_context *, fz_document *);