gl-renderer: drop redundant texture lookups

Do not call texture2D() in the shader when we already have the result.
Simpler code, maybe even a little bit faster?

Suggested-by: Harish Krupo <harishkrupo@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/renderer-gl/fragment.glsl

@@ -106,22 +106,18 @@ main()
 
 	} else if (c_variant == SHADER_VARIANT_Y_UV) {
 		yuva.x = texture2D(tex, v_texcoord).x;
-		yuva.y = texture2D(tex1, v_texcoord).r;
+		yuva.yz = texture2D(tex1, v_texcoord).rg;
-		yuva.z = texture2D(tex1, v_texcoord).g;
 		yuva.w = alpha;
 		gl_FragColor = yuva2rgba(yuva);
 
 	} else if (c_variant == SHADER_VARIANT_Y_XUXV) {
 		yuva.x = texture2D(tex, v_texcoord).x;
-		yuva.y = texture2D(tex1, v_texcoord).g;
+		yuva.yz = texture2D(tex1, v_texcoord).ga;
-		yuva.z = texture2D(tex1, v_texcoord).a;
 		yuva.w = alpha;
 		gl_FragColor = yuva2rgba(yuva);
 
 	} else if (c_variant == SHADER_VARIANT_XYUV) {
-		yuva.x = texture2D(tex, v_texcoord).b;
+		yuva.xyz = texture2D(tex, v_texcoord).bgr;
-		yuva.y = texture2D(tex, v_texcoord).g;
-		yuva.z = texture2D(tex, v_texcoord).r;
 		yuva.w = alpha;
 		gl_FragColor = yuva2rgba(yuva);