lovr.mirror disables blending when drawing headset texture;

This is so transparent headset textures (when rendering overlays)
don't ghost.  Alternatively, clear could be used.

src/resources/boot.lua

@@ -218,6 +218,8 @@ end
 
 function lovr.mirror()
   if lovr.headset then -- On some systems, headset module will be disabled
+    local blend, alpha = lovr.graphics.getBlendMode()
+    lovr.graphics.setBlendMode()
     local texture = lovr.headset.getMirrorTexture()
     if texture then    -- On some drivers, texture is printed directly to the window
       if lovr.headset.getDriver() == 'oculus' then
@@ -226,6 +228,7 @@ function lovr.mirror()
         lovr.graphics.fill(lovr.headset.getMirrorTexture())
       end
     end
+    lovr.graphics.setBlendMode(blend, alpha)
   else
     lovr.graphics.clear()
     lovr.draw()