Tech ENABLE_GLBEGIN_GLEND_SHADERS_ATTRIBUTES - Use vertex attributes and matrices in shaders. 1st installment.

Shader: flat - Default bed

(cherry picked from commit prusa3d/PrusaSlicer@a5ff37013b)

resources/shaders/flat_attr.vs

@@ -0,0 +1,10 @@
+#version 110
+
+attribute vec3 v_position;
+
+uniform mat4 projection_view_model_matrix;
+
+void main()
+{
+    gl_Position = projection_view_model_matrix * vec4(v_position, 1.0);
+}