Check type selection for sizes

[elfring]

How do you think about to replace any "int" for the following parameters by the data type "size_t"?

Examples:

• num_glyphs (structure member)
  
• width, max_ascent, max_descent (function "txfGetStringMetrics")
  
• len (functions "txfRenderString", "txfRenderFancyString", "copy_element_contents", "sgets")
  
• i (function "txf_render_string")

[elfring]

I would appreciate a comment for the reasons why a safer data type will not be introduced at the proposed places.