Add flags for vertical alignment of text in rectangles.

[davyjones]

Currently, HPDF_TextAlignment has LEFT, RIGHT, CENTER and JUSTIFY options. Would be nice to have TOP, BOTTOM and MIDDLE as well.

Commit 256dfd3 has the necessary changes.

Changed HPDF_Page_TextRect to support multiple flags. For example, to place text in the center-middle of a rectangle, HPDF_TALIGN_CENTER | HPDF_TALIGN_MIDDLE can be passed to HPDF_Page_TextRect.