Tech ENABLE_GL_IMGUI_SHADERS - Render imgui using shaders

(cherry picked from commit prusa3d/PrusaSlicer@d0d89a4d5b)

src/slic3r/GUI/GLShadersManager.cpp

@@ -33,6 +33,8 @@ std::pair<bool, std::string> GLShadersManager::init()
 
     bool valid = true;
 
+    // imgui shader
+    valid &= append_shader("imgui", { "imgui.vs", "imgui.fs" });
     // basic shader, used to render all what was previously rendered using the immediate mode
     valid &= append_shader("flat_attr", { "flat_attr.vs", "flat.fs" });
     // basic shader for textures, used to render textures