[NaCl SDK] Simplify graphics3d example.

native_client_sdk/src/examples/api/graphics_3d/vertex_shader_es2.vert

-uniform mat4 a_MVP;
-attribute vec2 a_texCoord; 
-attribute vec3 a_color; 
-attribute vec4 a_position; 
-varying  vec3 v_color; 
-varying  vec2 v_texCoord; 
-void main() 
-{
- gl_Position = a_MVP * a_position; 
- v_color = a_color;
- v_texCoord = a_texCoord;
-}