This reverts commit aff0eac2. The RGB
values are already alpha values, they don't need to be pre-multiplied.

The Star Trek: Armada II demo tries to install a Windows 95 winsock 
redist if the detected ws2_32.dll version is too old.

The blit_shader is activated before draw_textured_quad, so after use
it should be deactivated instead of being activated again.