cpattern:SetCornerColorRGBA(corner_num, red, green, blue, alpha)
The color is specified in the same way as in ccontext:SetSourceRGBA().
Valid values for corner_num are from 0 to 3 and identify the
corners as explained in cairo.PatternMesh().
Note: If the pattern is not a mesh pattern then it will be put
into an error status with a status of
#CAIRO_STATUS_PATTERN_TYPE_MISMATCH. If corner_num is not valid,
the pattern will be put into an error status with a status of
#CAIRO_STATUS_INVALID_INDEX. If the pattern has no current patch,
it will be put into an error status with a status of
#CAIRO_STATUS_INVALID_MESH_CONSTRUCTION.