bind_sampler(blend_program, uniform_image1_tex, 1, tex1, linear_sampler);
bind_sampler(blend_program, uniform_flow_tex, 2, flow_tex, linear_sampler); // May be upsampled.
glProgramUniform1f(blend_program, uniform_alpha, alpha);
bind_sampler(blend_program, uniform_image1_tex, 1, tex1, linear_sampler);
bind_sampler(blend_program, uniform_flow_tex, 2, flow_tex, linear_sampler); // May be upsampled.
glProgramUniform1f(blend_program, uniform_alpha, alpha);
glViewport(0, 0, level_width, level_height);
fbos.render_to(output_tex);
glViewport(0, 0, level_width, level_height);
fbos.render_to(output_tex);