implicits args -> error in Image.h

Also fix an apparent bug in operator()(int)
Clean up static_image
Merge branch 'master' of github.com:halide/Halide
CodeGen_GPU_Host now a template to generate both X86 and ARM variants.
Update INSTALL
Switch from preprocessor templating to C++ templating
Merge branch 'master' of github.com:halide/Halide
Update INSTALL
Fix for older compilers.
Merge branch 'gpu-host-targets'
Merge branch 'master' of github.com:halide/Halide
Add HALIDE_FUNCTION_ATTRS decorator to generated function prototypes

This allows (e.g.) __attribute__((warn_unused_result) to be specified
on Halide calls.