bunch of fixes to make it work with Nvidia Tegra K1

 - renamed texture variable in fragment shader to
   fakeTexture, so that it doesn't clash with
   texture lookup function
 - removed unused texDepth uniform
 - fix program switching - retrieve uniforms
 - avoid texture calls when texture is not loaded
   or when using simple fragment shader without
   texture
 - report OpenGL and GLSL versions when context is set