Enable layers in non-tile modes.

The current code is very messy as far as GFX goes. There are more
elegant ways of tackling this issue, but they are very difficult with
our current code.

The idea behind this issue is that the backgrounds are not switching
properly for the lcd. So we enable the layers accordingly to
`layerSettings & DISPCNT` on every `CPUUpdateRenderBuffers`.

The bits of `layerSettings` enconde each layer and objs.
We actually only need 2 of these, but I can't do much more here
unsupervised.

- Fix #376.

src/gba/GBA.cpp

@@ -577,6 +577,7 @@ extern uint32_t line3[240];
 
 void CPUUpdateRenderBuffers(bool force)
 {
+    layerEnable = layerSettings & DISPCNT;
     if (!(layerEnable & 0x0100) || force) {
         CLEAR_ARRAY(line0);
     }