Build an <a:solidFill> block with an sRGB color and an
alpha channel. Used inside DrawingML shape properties wrappers such
as <a:spPr>, <a:ln>, <a:rPr>, and chartEx-specific wrappers
such as <cx:spPr>.
This is a low-level helper intended for packages that emit OOXML
fragments (e.g. mschart, rvg). End users typically rely on
higher-level objects (fp_text(), fp_border(), sp_line()) that
embed the <a:solidFill> block themselves.