reportlab-4.2.0 can support bitmap transparency

[replabrobin]

If using reportlab[pycairo] (ie cairo backend) the bitmap output can use transparency. An example is in tests/test_graphics_images.py. It looks like this; we changed the background and backendFmt using d._renderPM_backendFmt = 'ARGB32'
and d._renderPM_bg = toColor('white').clone(alpha=0) (they could also be passed into save as extra keywords).

	@rlSkipUnless(rlPyCairo,'cannot import rlPyCairo')
	def test7(self):
		rw = 10
		rg = 10
		xgap = 5
		h = w = 3*rw+4*rg
		d = Drawing(2*h + xgap,h)
		C = toColor('red'),toColor('green'),toColor('blue')
		g = Group()

		def colconv(j,c):
			if j==0: return c
			if j==1: return CMYKColor(*rgb2cmyk(c.red,c.green,c.blue),alpha=c.alpha)

		from reportlab.graphics.widgets.grids import ShadedRect
		for i in range(3):
			c = C[i]
			sc = c.clone(alpha=0.1)
			ec = c.clone(alpha=1.0)
			for j in range(2):
				x = j*(w+xgap)
				d.add(ShadedRect(x=x,y=rg+(rg+rw)*i,width=w,height=rw,orientation='vertical',numShades=w//10,
						fillColorStart=colconv(j,sc),fillColorEnd=colconv(j,ec),
						strokeColor=None,strokeWidth=0))

				g.add(ShadedRect(x=x+rg+(rg+rw)*i,y=0,width=rw,height=h,orientation='horizontal',numShades=w//10,
						fillColorStart=colconv(j,sc),fillColorEnd=colconv(j,ec),
						strokeColor=None,strokeWidth=0))
		d.add(g)

		d._renderPM_backendFmt = 'ARGB32'
		d._renderPM_bg = toColor('white').clone(alpha=0)	#transparent white
		d.save(formats=['png'],
			fnRoot=outputfile(os.path.join("charts-out","test_graphics_images_alpha_test.png")), outDir=None)