Fix shadow artifacts by employing hardware clamping (#7532)

Author: BMagnu

code/def_files/data/effects/main-g.sdr

@@ -163,9 +163,6 @@ out VertexOutput {
 				gl_Position = shadow_proj_matrix[instanceID] * gl_in[vert].gl_Position;
 			#prereplace ENDIF_FLAG //MODEL_SDR_FLAG_TRANSFORM
 
-			if(gl_Position.z < -1.0)
-			gl_Position.z = -1.0;
-
 			vertOut.position = gl_in[vert].gl_Position;
 			vertOut.normal = vertIn[vert].normal;
 			vertOut.texCoord = vertIn[vert].texCoord;

code/graphics/opengl/gropengltnl.cpp

@@ -697,6 +697,8 @@ void gr_opengl_shadow_map_start(matrix4 *shadow_view_matrix, const matrix *light
 	glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
 	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
+	glEnable(GL_DEPTH_CLAMP);
+
 	Rendering_to_shadow_map = true;
 	Glowpoint_override_save = Glowpoint_override;
 	Glowpoint_override = true;
@@ -715,6 +717,8 @@ void gr_opengl_shadow_map_end()
 	if(!Rendering_to_shadow_map)
 		return;
 
+	glDisable(GL_DEPTH_CLAMP);
+
 	gr_end_view_matrix();
 	Rendering_to_shadow_map = false;