Adjust API: Environment is Copy; prepare returns bool

[dhardy]

This contains a few fixes and a few API tweaks.

• Remove fn line_is_ltr; add fn text_is_rtl
• Replace separate dpp, pt_size parameters with single dpem
• Remove PX_VALIGN configuration flag: there is probably no reason to ever turn this off
• Replace BIDI flag with more options under Direction enum
• Add measure_width, measure_height, bounding_box and vertically_align methods to allow more efficient size-deduction followed by alignment
• prepare* and update_env methods now only return a boolean describing whether the bounding box is exceeded