Add external texture support (GLES3)

Co-authored-by: Fredia Huya-Kouadio <fhuyakou@gmail.com>
Co-authored-by: Mauricio Narvaez <nvz@meta.com>

servers/rendering/shader_language.h

@@ -91,6 +91,7 @@ public:
 		TK_TYPE_USAMPLER3D,
 		TK_TYPE_SAMPLERCUBE,
 		TK_TYPE_SAMPLERCUBEARRAY,
+		TK_TYPE_SAMPLEREXT,
 		TK_INTERPOLATION_FLAT,
 		TK_INTERPOLATION_SMOOTH,
 		TK_CONST,
@@ -235,6 +236,7 @@ public:
 		TYPE_USAMPLER3D,
 		TYPE_SAMPLERCUBE,
 		TYPE_SAMPLERCUBEARRAY,
+		TYPE_SAMPLEREXT,
 		TYPE_STRUCT,
 		TYPE_MAX
 	};