Oculus Rift|Renderer: Separately configurable Oculus Rift multisampling

Added “rend-vr-rift-samples” to control the number of samples used
for the unwarped Oculus Rift framebuffer. The default is 2.
Merge branch 'ui-alerts' of ssh://github.com/skyjake/Doomsday-Engine into ui-alerts
Fixed|UI|Client: Button text hover color with rich formatting

Rich formatting would override the button hover color, which would
result in a potential clash with the hover background. Now the
hover text color is applied using a modulation color that is
applied in the vertex colors.
Cleanup