Sets the source pattern within cr to a translucent color. This
color will then be used for any subsequent drawing operation until
a new source pattern is set.
The color and alpha components are floating point numbers in the
range 0 to 1. If the values passed in are outside that range, they
will be clamped.
The default source pattern is opaque black, (that is, it is
equivalent to cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0)).
Sets the source pattern within cr to a translucent color. This color will then be used for any subsequent drawing operation until a new source pattern is set. The color and alpha components are floating point numbers in the range 0 to 1. If the values passed in are outside that range, they will be clamped. The default source pattern is opaque black, (that is, it is equivalent to cairo_set_source_rgba(cr, 0.0, 0.0, 0.0, 1.0)).