Add support for GL_EXT_YUV_target

Test/300samplerExternalYUV.frag

@@ -0,0 +1,38 @@
+#version 300 es
+
+#extension GL_EXT_YUV_target : enable
+
+uniform __samplerExternal2DY2YEXT sExt;
+precision mediump __samplerExternal2DY2YEXT;
+uniform __samplerExternal2DY2YEXT mediumExt;
+uniform highp __samplerExternal2DY2YEXT highExt;
+
+void main()
+{
+    texture2D(sExt, vec2(0.2));  // ERROR
+    texture2D(mediumExt, vec2(0.2));  // ERROR
+    texture2D(highExt, vec2(0.2));  // ERROR
+    texture2DProj(sExt, vec3(0.3));  // ERROR
+    texture2DProj(sExt, vec4(0.3));  // ERROR
+
+    int lod = 0;
+    highp float bias = 0.01;
+    textureSize(sExt, lod);
+    texture(sExt, vec2(0.2));
+    texture(sExt, vec2(0.2), bias);
+    textureProj(sExt, vec3(0.2));
+    textureProj(sExt, vec3(0.2), bias);
+    textureProj(sExt, vec4(0.2));
+    textureProj(sExt, vec4(0.2), bias);
+    texelFetch(sExt, ivec2(4), lod);
+
+    texture3D(sExt, vec3(0.3));  // ERROR
+    texture2DProjLod(sExt, vec3(0.3), 0.3);  // ERROR
+    texture(sExt, vec3(0.3));  // ERROR
+    textureProjLod(sExt, vec3(0.3), 0.3);  // ERROR
+}
+
+#extension GL_EXT_YUV_target : disable
+
+uniform __samplerExternal2DY2YEXT badExt;  // ERROR
+