Cleanup: Put GPU_state_init inside gpu_state.c

Also put glDisable(GL_DITHER) in it since we don't even use it (but is
enabled by default).

Also leave GL_MULTISAMPLE on by default since it has no impact on non-MSAA
framebuffers.

source/blender/gpu/intern/gpu_framebuffer.c

@@ -541,10 +541,6 @@ void GPU_framebuffer_bind(GPUFrameBuffer *fb)
   }
 #endif
 
-  if (fb->multisample) {
-    glEnable(GL_MULTISAMPLE);
-  }
-
   glViewport(0, 0, fb->width, fb->height);
 }