[xft]remove unused variable 'point' to fix -Wunused-but-set-variable

[cg]cleanup some code, add some space

[libcg]remove unused font_size from state

[libcg]using float for libcg

[screenshots]update screenshots, some commit f924651b85cac47dbe15f51a…

…4aa320461fc1d07b

[libcg]fix arc mathematics and update copyright

[examples]remove cg_clip for test_image

[libcg]fix blend_transformed_argb

[libcg]move some macro to cg.h

[examples]add clip image's rectangle