DefaultMaterial: Actually use a screen blend mode when requested

The Screen blendmode is defined as:
1-(1-SRC)*(1-DST)
Which is the same as:
SRC + DST(1-SRC) or in glBlendFunc terms:
GL_ONE, GL_ONE_MINUS_SRC_COLOR

Output should now be the expected result, but should break the lancelot
test.

Fixes: QTBUG-77786
Change-Id: I01ef0b1e08389394f1a6dc7019587a3473f22f5f
Reviewed-by: Christian Strømme <christian.stromme@qt.io>