add clear_mark to clear only selected channels in the framebuffer

src/rtt.h

@@ -27,6 +27,7 @@ public:
     bool create(int width, int height, int tex = -1, GLint internal_format = GL_RGBA8, bool depth_buffer = false);
     bool recreate() { return create(w, h); }
     void clear(glm::vec4 color = glm::vec4(0));
+    void clear_mask(glm::bool4 mask, glm::vec4 color = glm::vec4(0));
     glm::ivec4 calc_bounds();
     uint8_t* readTextureData(uint8_t* buffer = nullptr);
     float* readTextureDataFloat(float* buffer = nullptr);