Fix imagickpixeliterator_class.c:427:2: warning: pointer targets in passing argument 2 of 'PixelGetCurrentIteratorRow' differ in signedness [-Wpointer-sign]

This is "unsigned long" in until 6.7
This is "size_t" since 6.7
Fix imagickdraw_class.c:507:16: warning: unused variable 'error' [-Wunused-variable]
Avoid deprecated call, fix build warning

From deprecated.c

WandExport void DrawSetFillAlpha(DrawingWand *wand,const double fill_alpha)
{
  DrawSetFillOpacity(wand,fill_alpha);
}

WandExport void DrawSetStrokeAlpha(DrawingWand *wand,const double stroke_alpha)
{
  DrawSetStrokeOpacity(wand,stroke_alpha);
}