Add shader stage for final screen texture draw

src/hardware/hw_defs.h

@@ -166,6 +166,7 @@ enum
 	SHADER_SKY,
 	SHADER_PALETTE_POSTPROCESS,
 	SHADER_UI_COLORMAP_FADE,
+	SHADER_FINAL_POST_PROCESS,
 
 	NUMSHADERTARGETS
 };

src/hardware/hw_shaders.c

@@ -435,6 +435,8 @@ customshaderxlat_t shaderxlat[] =
 	{"Sky", SHADER_SKY},
 	{"PalettePostprocess", SHADER_PALETTE_POSTPROCESS},
 	{"UIColormapFade", SHADER_UI_COLORMAP_FADE},
+	{"FinalPostProcess", SHADER_FINAL_POST_PROCESS},
+
 	{NULL, 0},
 };
 

src/hardware/r_opengl/r_opengl.c

@@ -3449,6 +3449,8 @@ void GL_DrawScreenFinalTexture(int tex, int width, int height)
 	GL_ClearBuffer(true, false, false, &clearColour);
 	pglBindTexture(GL_TEXTURE_2D, screenTextures[tex]);
 
+	Shader_SetUniforms(NULL, NULL, NULL, NULL); // prepare shader, if it is enabled
+
 	pglColor4ubv(white);
 
 	pglTexCoordPointer(2, GL_FLOAT, 0, fix);

src/sdl/ogl_sdl.c

@@ -185,7 +185,9 @@ void OglSdlFinishUpdate(boolean waitvbl)
 	SDL_GetWindowSizeInPixels(window, &sdlw, &sdlh);
 
 	HWR_MakeScreenFinalTexture();
+	GL_SetShader(HWR_GetShaderFromTarget(SHADER_FINAL_POST_PROCESS));
 	HWR_DrawScreenFinalTexture(sdlw, sdlh);
+	GL_UnSetShader();
 	SDL_GL_SwapWindow(window);
 
 	GL_GClipRect(0, 0, vid.width, vid.height, NZCLIP_PLANE);