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

Includes the following spirv-opt improvements:

Preserve inst-to-block and def-use in passes.
Add store for var initializer in inlining.
Handle types with self references.

Relax a stringToDouble test for, OSX AppleClang 9.1

1e-323 was flushed to zero.
1e-308 is also flushed to zero.
Use 1e-307 instead, which still satisfies the test intent.

Fixes #1400

The major version number was bumped in the previous commit to support
this.

Fix several signed/unsigned comparison compile warnings.

Update spirv-tools known-good.