anv/blit2d: Use texel fetch in frag shader

anv/blit2d: Use texel fetch in frag shader

The texelFetch operation requires that the sampled texture coordinates
be unnormalized integers. This will simplify the copy shader for
w-tiled images (stencil buffers).

v2 (Jason):
   Use f2i for texel coords
   Fix num_components indirectly
   Use float inputs for interpolation
   Nest tex_pos functions

Suggested-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>