(API Change) Remove obs_graphics()

API Removed:
- graphics_t obs_graphics();
Replaced With:
- void obs_enter_graphics();
- void obs_leave_graphics();

Description:
  obs_graphics() was somewhat of a pointless function.  The only time
that it was ever necessary was to pass it as a parameter to
gs_entercontext() followed by a subsequent gs_leavecontext() call after
that.  So, I felt that it made a bit more sense just to implement
obs_enter_graphics() and obs_leave_graphics() functions to do the exact
same thing without having to repeat that code.  There's really no need
to ever "hold" the graphics pointer, though I suppose that could change
in the future so having a similar function come back isn't out of the
question.

Still, this at least reduces the amount of unnecessary repeated code for
the time being.

obs/window-basic-main.cpp

@@ -491,7 +491,7 @@ void OBSBasic::InitOBSCallbacks()
 
 void OBSBasic::InitPrimitives()
 {
-	gs_entercontext(obs_graphics());
+	obs_enter_graphics();
 
 	gs_renderstart(true);
 	gs_vertex2f(0.0f, 0.0f);
@@ -508,7 +508,7 @@ void OBSBasic::InitPrimitives()
 	}
 	circle = gs_rendersave();
 
-	gs_leavecontext();
+	obs_leave_graphics();
 }
 
 void OBSBasic::OBSInit()
@@ -588,10 +588,10 @@ OBSBasic::~OBSBasic()
 	ui->sources->clear();
 	ui->scenes->clear();
 
-	gs_entercontext(obs_graphics());
+	obs_enter_graphics();
 	vertexbuffer_destroy(box);
 	vertexbuffer_destroy(circle);
-	gs_leavecontext();
+	obs_leave_graphics();
 
 	obs_shutdown();