Tech ENABLE_LEGACY_OPENGL_REMOVAL - porting remaining changes

(cherry picked from commit prusa3d/PrusaSlicer@2f572d3cf0 )

resources/shaders/flat_texture_attr.vs

@@ -0,0 +1,15 @@
+#version 110
+
+attribute vec3 v_position;
+attribute vec2 v_tex_coord;
+
+uniform mat4 view_model_matrix;
+uniform mat4 projection_matrix;
+
+varying vec2 tex_coord;
+
+void main()
+{
+	tex_coord = v_tex_coord;
+    gl_Position = projection_matrix * view_model_matrix * vec4(v_position, 1.0);
+}