Uses This == dstSwapchain->frontBuffer instead as suggested by Henri Verbeet.

Handled in ActivateContext, must be remains of pre-context management code.

This prevents shader path from being entered for an offscreen surface
when there is p8 render target and fixes failures in ddraw visual test
(with opengl rendering and RTL_READDRAW mode) and visual glitches in
Red Alert.

wined3d/d3d9: Set the initial scissorrect to the dimesions of the backbuffer from the first swapchain of the device.