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