[glsl-out] Support push constant emulation

[Herschel]

gfx/wgpu ostensibly supports emulating push constants using uniform buffers on non-Vulkan backends, including GL. However, naga currently errors when translating a shader to GLSL using push constants with PushConstantNotSupported. Ideally the shader is translated to use uniforms the way that gfx/wgpu expects.

Sample shader:
color.vert.zip

cargo run --features spv-in,glsl-out -- color.frag.spv color.out.frag

    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running `target\debug\naga.exe color.vert.spv color.vert.frag`
Push constants aren't supported:
error: process didn't exit successfully: `target\debug\naga.exe color.frag.spv color.out.frag` (exit code: 1)

Sample repo using wgpu-rs:

git clone -b gl https://github.com/Herschel/ruffle
cargo run --package ruffle_desktop -- -g gl magicaltrevor.swf

wgpu error: Validation Error

Caused by:
    In Device::create_render_pipeline
    Internal error in stage VERTEX: Shader module failed to compile: PushConstantNotSupported shader compilation failed: Vertex


thread 'main' panicked at 'Handling wgpu errors as fatal by default', /home/mwelsh/.cargo/git/checkouts/wgpu-rs-40ea39809c03c5d8/80fdbd2/src/backend/direct.rs:1942:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted (core dumped)

naga @ 93e57ff

[JCapucho]

Fixed in #1683