Feature/image view tex coords

[adielfernandez]

Add in overloaded setTexture() method for specifying the texture coordinates of the texture subsection ImageView should draw.

Also includes getters and setters for accessing the current tex coords and the default (non-static) tex coords.

NOTES:

• Calling setTexCoords() causes re-allocation of the batch so it's too slow to continuously update texcoords in realtime.
• Shader used to draw Image is static, batch is now non-static. Performance seems to be same as previous implementation, but more testing is welcome.