A fix to bug preventing to use textures in users shaders for derived class

[stephane2017hongkong]

Hi,

In SampleScene example, the shaders are not using textures.
In the code I wrote, I do use a texture.
It causes a problem because of unspecified use of texture.
A fix is , within the HoloPlayContext.cpp, to specify a texture to use

Here is a work around that worked for me :
Replace

// 8. now render on view texture
glBindVertexArray(viewTex);

by

glActiveTexture(GL_TEXTURE0)
glBindTexture(GL_TEXTURE_2D, self.viewTexture)

Then in the derived class from HoloPlayContext, specifiy

glActiveTexture(GL_TEXTURE1)

Allows to have the result compiling and display nicely on the LookingGlass.

I hope it helps.