Add GLSL linter using glslang (#914)

* Add a glslang linter for GLSL

test/command_callback/test_glslang_command_callback.vader

@@ -0,0 +1,42 @@
+Before:
+  Save g:ale_glsl_glslang_executable
+  Save g:ale_glsl_glslang_options
+
+  unlet! g:ale_glsl_glslang_executable
+  unlet! g:ale_glsl_glslang_options
+
+  runtime ale_linters/glsl/glslang.vim
+  call ale#test#SetDirectory('/testplugin/test/command_callback')
+
+After:
+  Restore
+  unlet! g:ale_cuda_nvcc_executable
+  unlet! g:ale_cuda_nvcc_options
+  call ale#linter#Reset()
+
+Execute(Executable should default to glslangValidator):
+  AssertEqual
+  \ 'glslangValidator',
+  \ ale_linters#glsl#glslang#GetExecutable(bufnr(''))
+
+Execute(Executable should be configurable):
+  let g:ale_glsl_glslang_executable = 'foobar'
+  AssertEqual
+  \ 'foobar',
+  \ ale_linters#glsl#glslang#GetExecutable(bufnr(''))
+
+Execute(Command should use executable):
+  AssertEqual
+  \ 'glslangValidator  -C %t',
+  \ ale_linters#glsl#glslang#GetCommand(bufnr(''))
+
+  let g:ale_glsl_glslang_executable = 'foobar'
+  AssertEqual
+  \ 'foobar  -C %t',
+  \ ale_linters#glsl#glslang#GetCommand(bufnr(''))
+
+Execute(Options should work):
+  let g:ale_glsl_glslang_options = '--test'
+  AssertEqual
+  \ 'glslangValidator --test -C %t',
+  \ ale_linters#glsl#glslang#GetCommand(bufnr(''))

test/handler/test_glslang_handler.vader

@@ -0,0 +1,21 @@
+Execute(The glsl glslang handler should parse lines correctly):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 4,
+  \     'col': 0,
+  \     'type': 'E',
+  \     'text': '''gl_ModelViewProjectionMatrix'' : undeclared identifier',
+  \   },
+  \   {
+  \     'lnum': 121,
+  \     'col': 0,
+  \     'type': 'W',
+  \     'text': '''switch'' : last case/default label not followed by statements',
+  \   },
+  \ ],
+  \ ale_linters#glsl#glslang#Handle(bufnr(''), [
+  \ 'ERROR: 0:4: ''gl_ModelViewProjectionMatrix'' : undeclared identifier',
+  \ 'WARNING: 0:121: ''switch'' : last case/default label not followed by statements',
+  \ 'ERROR: 2 compilation errors.  No code generated.',
+  \ ])