LibPDF: Don't crash on fixed-width type 1 fonts that use /MissingWidth

Type 1 fonts usually have a m_font_program and no m_font -- they only
have m_font if we're using a replacement font for the fonts that
were built-in to PDFs before Acrobat 4.0 (and must still work to
show existing files).

However, SimpleFont::get_glyph_width() used to always return a
float, which in Type1Font was only implemented if m_font was set.

Per spec, we're supposed to just use /MissingWidth for fonts that
are missing an entry in the descriptor's /Width array. However, for
built-in fonts, no explicit /Width array is needed (PDF 1.7 spec,
Appendix H.3, 5.5.1). So if we just always use /MissingWidth,
then PDFs that use a built-in font draw all their text on top
of each other (e.g. 000333.pdf from stillhq.com-pdfdb).

So change get_glyph_width() to return Optional<float>, return
it only in Type1Font if m_font is set, and use MissingWidth
if it isn't set.

That way, replacement fonts still return a width, and real
fonts that are supposed to have /Width and use /MissingWidth
for missing entries do what they're supposed to too, instead
of crashing.

From 20 (6%) to 16 (5%) crashes on the 300 first PDFs, and from
39 (7.8%) to 31 (6.2%) on the 500-random PDFs test.

Userland/Libraries/LibPDF/Fonts/SimpleFont.cpp

@@ -53,8 +53,10 @@ PDFErrorOr<Gfx::FloatPoint> SimpleFont::draw_string(Gfx::Painter& painter, Gfx::
         float glyph_width;
         if (auto width = m_widths.get(char_code); width.has_value())
             glyph_width = font_size * width.value() / 1000.0f;
+        else if (auto width = get_glyph_width(char_code); width.has_value())
+            glyph_width = width.value();
         else
-            glyph_width = get_glyph_width(char_code);
+            glyph_width = m_missing_width;
 
         draw_glyph(painter, glyph_position, glyph_width, char_code, paint_color);