add color dodge and multiply blending modes shaders, change tmp framebuffer to RGBA32F for better precision and fix the blending problem for the stroke not reaching full opacity

engine/rtt.h

@@ -15,7 +15,7 @@ public:
     ~RTT();
 
     void destroy();
-    bool create(int width, int height, int tex = -1);
+    bool create(int width, int height, int tex = -1, GLint internal_format = GL_RGBA8);
     bool recreate() { return create(w, h); }
     void clear(glm::vec4 color = glm::vec4(0));
     void readTextureData(uint8_t* buffer);