Optimize SPIR-V code

[GabrielMajeri]

This pull request adds an additional tool (spirv-opt) to the shader building pipeline. It is used to optimize the shader modules even further.

glslangValidator is good at compiling code, but it is meant to be a reference compiler, not an optimizer, therefore it lacks advanced optimizations.

The SPIRV-Tools repository contains a tool called spirv-opt which can be used to optimize shader binaries. I haven't run any advanced benchmarks, but on the examples, this tool reduces shader code size by about 33% (by eliminating dead / unnecessary code).

This will also open the way to even more advanced optimizations as spirv-opt gets improved.

The pull request includes the x64-Release Windows binary from CI, available here. I understand if you want to remove it and add it yourself since I could've potentialy included malicious code.

In order to enable to use additional tools I had to change glsl-to-spirv. I refactored both the build.rs and the lib.rs files, it should be easier to chain more tools in the future.

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To do:

• Fix building on Linux. CI works now.
• Test this on more code to make sure it works. The examples seem to be fine, and I enabled no "unsafe" optimizations, but I don't know if everything will still work.
• I tried adding the --strip-debug flag but it prevents shader reflection from working. @tomaka do you want to investigate or is it alright to merge without it? Won't enable it, unnecessary.

[tomaka]

I tried adding the --strip-debug flag but it prevents shader reflection from working. @tomaka do you want to investigate or is it alright to merge without it?

I suppose that --strip-debug does not add any further optimization, so I'm fine with merging without it.

[GabrielMajeri]

@tomaka It removes some metadata required for reflection, but I think it's OK, since the driver would throw it away anyway.

There are other options we could enable over time, described in this white paper.

[rukai]

glsl-to-spirv is no more #947

[antiagainst]

Related to this, since we already include the whole copy of SPIRV-Tools in Shaderc, it should be natural to extend the current shaderc-rs API to also expose SPIRV-Tools ones, if there is a need.