FFI: Blitbuffers, Freetype

This implements blitbuffer operations in FFI. *Warning*: We can not drop the libkoreader-blitbuffer.so yet, as it is still used for the operations writing to the framebuffer / SDL pane. When that part is converted, too, we can finally drop it. Most operations, however, are being done by the new code.

Speed test for blitting show it running at the same speed for blitting to even X coordinates and twice the speed for rendering to odd coordinates.

Freetype font rendering is converted to FFI, too. That fully replaces the libkoreader-ft.so.

Also creates an ffi-cdecl directory that contains .c files used for generating ffi declarations (e.g. ffi/freetype_h.lua)