++DFB: Add premultiplied caps to DFBApp
API: Add DSTF_NONE
IDirectFB: For windowed primary, set DWOP_ALPHACHANNEL when DSCAPS_PREMULTIPLIED is used (assuming blending)
IDirectFBSurface: Add DSTF_BLEND_FUNCS to keep custom blend funcs set, but follow blitting flags of font
dfbtest_font_blend: Add test for DSTF_BLEND_FUNCS