libobs: Add functions to push/pop blend states

This is particularly important for the filter pipeline in order to
ensure that when the last filter is reached that the original blend
state is properly reset.

libobs/graphics/graphics-internal.h

@@ -292,4 +292,5 @@ struct graphics_subsystem {
 	volatile long          ref;
 
 	struct blend_state     cur_blend_state;
+	DARRAY(struct blend_state) blend_state_stack;
 };

libobs/graphics/graphics.c

@@ -213,6 +213,7 @@ void gs_destroy(graphics_t *graphics)
 	pthread_mutex_destroy(&graphics->effect_mutex);
 	da_free(graphics->matrix_stack);
 	da_free(graphics->viewport_stack);
+	da_free(graphics->blend_state_stack);
 	if (graphics->module)
 		os_dlclose(graphics->module);
 	bfree(graphics);
@@ -1002,6 +1003,31 @@ void gs_perspective(float angle, float aspect, float near, float far)
 			ymin, ymax, near, far);
 }
 
+void gs_blend_state_push(void)
+{
+	graphics_t *graphics = thread_graphics;
+	if (!graphics) return;
+
+	da_push_back(graphics->blend_state_stack, &graphics->cur_blend_state);
+}
+
+void gs_blend_state_pop(void)
+{
+	graphics_t *graphics = thread_graphics;
+	struct blend_state *state;
+
+	if (!graphics) return;
+
+	state = da_end(graphics->blend_state_stack);
+	if (!state)
+		return;
+
+	gs_enable_blending(state->enabled);
+	gs_blend_function(state->src, state->dest);
+
+	da_pop_back(graphics->blend_state_stack);
+}
+
 void gs_reset_blend_state(void)
 {
 	graphics_t *graphics = thread_graphics;

libobs/graphics/graphics.h

@@ -537,6 +537,8 @@ EXPORT void gs_cubetexture_set_image(gs_texture_t *cubetex, uint32_t side,
 
 EXPORT void gs_perspective(float fovy, float aspect, float znear, float zfar);
 
+EXPORT void gs_blend_state_push(void);
+EXPORT void gs_blend_state_pop(void);
 EXPORT void gs_reset_blend_state(void);
 
 /* -------------------------- */