screenSpace: make it work when GL_EXT_gpu_shader4 is missing

Author: illwieckz

src/engine/renderer/glsl_source/screenSpace_vp.glsl

@@ -34,8 +34,16 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 /* screenSpace_vp.glsl */
 
+#if defined(HAVE_EXT_gpu_shader4)
 const vec2 vertices[3] = vec2[3] ( vec2( -1.0f, -1.0f ), vec2( 3.0f, -1.0f ), vec2( -1.0f, 3.0f ) );
 
 void main() {
 	gl_Position = vec4( vertices[gl_VertexID], 0.0f, 1.0f );
 }
+#else
+IN vec3 attr_Position;
+
+void main() {
+	gl_Position = vec4( attr_Position, 1.0f );
+}
+#endif

src/engine/renderer/tr_surface.cpp

@@ -531,13 +531,21 @@ void Tess_AddCubeWithNormals( const vec3_t position, const vec3_t minSize, const
 void Tess_InstantScreenSpaceQuad() {
 	GLIMP_LOGCOMMENT( "--- Tess_InstantScreenSpaceQuad ---" );
 
-	tr.skipVBO = true;
-
-	Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
-	rb_surfaceTable[Util::ordinal( *( tr.genericTriangle->surface ) )]( tr.genericTriangle->surface );
-	Tess_DrawElements();
-
-	tr.skipVBO = false;
+	if ( glConfig2.gpuShader4Available )
+	{
+		tr.skipVBO = true;
+		Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
+		rb_surfaceTable[Util::ordinal( *( tr.genericTriangle->surface ) )]( tr.genericTriangle->surface );
+		Tess_DrawElements();
+		tr.skipVBO = false;
+	}
+	else
+	{
+		Tess_Begin( Tess_StageIteratorDummy, nullptr, true, -1, 0 );
+		rb_surfaceTable[Util::ordinal( *( tr.genericQuad->surface ) )]( tr.genericQuad->surface );
+		GL_VertexAttribsState( ATTR_POSITION );
+		Tess_DrawElements();
+	}
 
 	GL_CheckErrors();
 

src/engine/renderer/tr_vbo.cpp

@@ -555,8 +555,12 @@ void R_BindNullIBO()
 
 static void R_InitGenericVBOs() {
 	// Min and max coordinates of the quad
-	static const vec3_t min = { 0.0f, 0.0f, 0.0f };
+
+	static const vec3_t min_new = { 0.0f, 0.0f, 0.0f };
+	static const vec3_t min_old = { -1.0f, -1.f, -1.0f };
+	const float *min = glConfig2.gpuShader4Available ? min_new : min_old;
 	static const vec3_t max = { 1.0f, 1.0f, 0.0f };
+
 	{
 		/*
 			Quad is a static mesh with 4 vertices and 2 triangles