Tech ENABLE_GLBEGIN_GLEND_REMOVAL - 1st installment - Selection bounding box

(cherry picked from commit prusa3d/PrusaSlicer@22f38235ea)

resources/shaders/flat.fs

@@ -0,0 +1,8 @@
+#version 110
+
+uniform vec4 uniform_color;
+
+void main()
+{
+    gl_FragColor = uniform_color;
+}

resources/shaders/flat.vs

@@ -0,0 +1,6 @@
+#version 110
+
+void main()
+{
+    gl_Position = ftransform();
+}