Fix chat opacity

This patch resinerts a GL11.glEnable(GL11.GL_BLEND) call that was present in Minecraft in 1.6.4 yet absent in Minecraft 1.7.x.
As a result of that absence, the chat opacity slider under multiplayer settings is nonfunctional - it changes the opacity of the gray chat background, but not the opacity of the chat words themselves.
This patch should have no side-effects, as it merely restores proper behavior from 1.6.4.

See: https://bugs.mojang.com/browse/MC-36812

To test: Type a message into chat, then slide the Chat Opacity slider around. Without the patch, only the gray background is faded in and out. With the patch, the words of the message are faded as well.