test-shader: The tex_coord varying is now an array under GLES2

Since the GLES2 wrapper grew support for multi-texturing, the
tex_coord varying variable defined in the vertex shader is actually an
array of texture coordinates so it ought to match in the fragment
shader in test-shader. This seemed to work anyway under Mesa/Intel but
under NVidia it does not so I don't think it's safe to assume that
linking a non-array varying with an array will work.

tests/interactive/test-shader.c

@@ -24,9 +24,9 @@ typedef struct
 
 #define GLES2_VARS \
   "precision mediump float;\n" \
-  "varying vec2 tex_coord;\n" \
+  "varying vec2 tex_coord[1];\n" \
   "varying vec4 frag_color;\n"
-#define TEX_COORD "tex_coord"
+#define TEX_COORD "tex_coord[0]"
 #define COLOR_VAR "frag_color"
 
 #else /* COGL_HAS_GLES2 */