remove half_texpixel in canvas since no longer used

drivers/gles3/shaders/canvas.glsl

@@ -381,7 +381,6 @@ void main() {
 
 	if (clip_rect_uv) {
 
-		vec2 half_texpixel = color_texpixel_size * 0.5;
 		uv = clamp(uv,src_rect.xy,src_rect.xy+abs(src_rect.zw));
 	}