pango: update to 1.15.1, changes LIBV to 1.6.0

pango: update to 1.15.1, changes LIBV to 1.6.0
from the NEWS:
* Optimizations:
  - Rework PangoLayout algorithms to avoid calling a recursive call to
    pango_layout_get_extents().  Avoids one pango_font_get_glyph_extents()
    call per glyph per layout rendering.  We now make 2 such calls.
  - Don't cache run_logical_rect in PangoLayoutIter.
    Avoids one pango_font_get_glyph_extents() call per glyph per layout
    rendering.  We are down to 1 now.
  - Cache line extents.  This avoids the final pango_font_get_glyph_extents()
    call per glyph per layout rendering.  Now we don't make any
    pango_font_get_glyph_extents() calls when rendering a layout (some
    conditions apply)!
  - Replace PANGO_IS_RENDERER with PANGO_IS_RENDERER_FAST for most
    internal uses, that is, just a NULL-check.
  - Use simple, fast, casts instead of PANGO_CAIRO_*() macros where the
    object cannot have reached us if it wasn't of the right type.