Added proper font-weight and font-style handling with @font-face rule.

This change fixes some issues when specifying a specific font-style or
font-weight through @font-face. For example, when importing a special font
resource specifically for the italic font-style, the PDF output would not
match the correct font if the css rule specifies the style/weight.

Example from my stylesheet:

```css
@font-face {
  font-family: "Eurostile LT";
  font-weight: normal;
  src: asset-url("pdf/fonts/eurostile.ttf");
  -fs-pdf-font-embed: embed;
  -fs-pdf-font-encoding: Identity-H;
}

@font-face {
  font-family: "Eurostile LT";
  src: asset-url("pdf/fonts/eurostile_bold.ttf");
  font-weight: bold;
  -fs-pdf-font-embed: embed;
  -fs-pdf-font-encoding: Identity-H;
}

body {
   font-family: "Eurostile LT";
   font-weight: bold;
}
```

Without this patch the PDF would not use the bold font.