OGL: implement bounding box support with ssbo

This implemention tries to be as accurate as the old SW implemention, but it will remove the dependcy of our vertexloader on videosw.

Source/Core/VideoCommon/PixelShaderGen.h

@@ -61,7 +61,7 @@ struct pixel_shader_uid_data
 	u32 per_pixel_depth : 1;
 	u32 forced_early_z : 1;
 	u32 early_ztest : 1;
-	u32 pad1 : 1;
+	u32 bounding_box : 1;
 
 	u32 texMtxInfo_n_projection : 8; // 8x1 bit
 	u32 tevindref_bi0 : 3;