Fix warnings in st_background_effect.c

Add <math.h> to fix implicit declaration of round()

Add extern definitions for cogl_pipeline_set_uniform_1i(),
cogl_pipeline_set_layer_combine()

Add explicit cast of vertical_FRO to CoglFramebuffer* type (this is a
baseclass conversion which is valid and fixes a warn-as-error issue when
building).
Merge branch 'st-background-effect-cleanup' of https://github.com/wrouesnel/Cinnamon