CairoPattern along the line defined
by (x0, y0) and (x1, y1). Before using the gradient pattern, a
number of color stops should be defined using
cairoPatternAddColorStopRgb or
cairoPatternAddColorStopRgba.cairoPatternCreateLinear(x0, y0, x1, y1)CairoPattern] the newly created CairoPattern if successful, or
an error pattern in case of no memory.This function will always return a valid pointer, but if an error
occurred the pattern status will be set to an error. To inspect
the status of a pattern use cairoPatternStatus.
cairoPatternSetMatrix.