NDK build: Use NDK r17b and its own CMake toolchain file

More recent NDK releases have their own CMake toolchain file.
Use it.

Also, download the NDK from github.com:dneto0/android-ndk.
That is a fork of the repo we used to use, but we have more
control over how long it stays stable.

WIP: Travis-CI: Use Android NDK r13b specifically   

The Travis-CI bot downloads a copy of the Android NDK.  The source
we get it from recently updated to Android NDK r17b.  However,
the android.toolchain.cmake file does not know how to parse the
Android native API level from that version of the NDK.  So check
out the NDK r13b version that we were using until yesterday.

Fixes #1439

Reflection exposes the Shader Stages where a Uniform is present

Use public SPIRV-Tools header

Implement GL_EXT_shader_16bit_storage and GL_EXT_shader_8bit_storage …

These introduce limited support for 8/16-bit types such that they can only be accessed in buffer memory and converted to/from 32-bit types.

Contributed from Khronos-internal work.

(If there is a bias issue, we need to discover what it is.)

Add support for GL_EXT_samplerless_texture_functions

SPV: Add option to print disassembly in standard form using SPIRV-Tools.

This results in better error recovery, including fewer
crashes on badly formed PP input.

Constant.cpp will throw a floating point divide by zero if floating point exceptions are enabled in Win32 causing the program to crash. This fix manually checks the right-hand argument of the division and sets appropriate Infinity, Negative Infinity, or NAN as if the floating point exceptions were disabled.

Include spirv-remap.exe in AppVeyor artifacts

script: Improve update sources script

- remove unused variable to pass pylint
- Use another approach to detect if known-good remote is already
  present to avoid the need for "ignore following errors" message.

Build: Make literal casting have fewer warnings and be more consistent.

Add support for OpConstantNull and OpConstantSampler to spirv-remap

Fixes issue #1408

This reverts commit ebec9094.

Khronos decided glslang was originally correct, and the specifications
are incorrect.

Take into account arrays of opaque types with --auto-map-bindings

TDefaultIoResolverBase::reserveSlot and getFreeSlot now have a size
parameter to reserve a range of bindings. This is used by
TDefaultIoResolver::resolveBinding to reserve a continuous range when
the type is an array and the target API is GL.

Update spirv-tools known-good