add post-effect-slot resource & add gfx.posteffects library

src/graphics/core/PostProcessing.cpp

@@ -9,7 +9,8 @@
 
 #include <stdexcept>
 
-PostProcessing::PostProcessing() {
+PostProcessing::PostProcessing(size_t effectSlotsCount)
+    : effectSlots(effectSlotsCount) {
     // Fullscreen quad mesh bulding
     float vertices[] {
         -1.0f, -1.0f, -1.0f, 1.0f, 1.0f,  1.0f,
@@ -77,6 +78,14 @@ void PostProcessing::render(
     }
 }
 
+void PostProcessing::setEffect(size_t slot, std::shared_ptr<PostEffect> effect) {
+    effectSlots.at(slot) = std::move(effect);
+}
+
+PostEffect* PostProcessing::getEffect(size_t slot) {
+    return effectSlots.at(slot).get();
+}
+
 std::unique_ptr<ImageData> PostProcessing::toImage() {
     return fbo->getTexture()->readData();
 }