Add GLSL linter using glslang (#914)

* Add a glslang linter for GLSL

ale_linters/glsl/glslang.vim

@@ -0,0 +1,46 @@
+" Author: Sven-Hendrik Haase <svenstaro@gmail.com>
+" Description: glslang-based linter for glsl files
+"
+" TODO: Once https://github.com/KhronosGroup/glslang/pull/1047 is accepted,
+" we can use stdin.
+
+let g:ale_glsl_glslang_executable =
+\ get(g:, 'ale_glsl_glslang_executable', 'glslangValidator')
+
+let g:ale_glsl_glslang_options = get(g:, 'ale_glsl_glslang_options', '')
+
+function! ale_linters#glsl#glslang#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'glsl_glslang_executable')
+endfunction
+
+function! ale_linters#glsl#glslang#GetCommand(buffer) abort
+    return ale_linters#glsl#glslang#GetExecutable(a:buffer)
+    \   . ' ' . ale#Var(a:buffer, 'glsl_glslang_options')
+    \   . ' ' . '-C %t'
+endfunction
+
+function! ale_linters#glsl#glslang#Handle(buffer, lines) abort
+    " Matches patterns like the following:
+    "
+    " ERROR: 0:5: 'foo' : undeclared identifier
+    let l:pattern = '^\(.\+\): \(\d\+\):\(\d\+\): \(.\+\)'
+    let l:output = []
+
+    for l:match in ale#util#GetMatches(a:lines, l:pattern)
+        call add(l:output, {
+        \   'lnum': str2nr(l:match[3]),
+        \   'col': str2nr(l:match[2]),
+        \   'text': l:match[4],
+        \   'type': l:match[1] is# 'ERROR' ? 'E' : 'W',
+        \})
+    endfor
+
+    return l:output
+endfunction
+
+call ale#linter#Define('glsl', {
+\   'name': 'glslang',
+\   'executable_callback': 'ale_linters#glsl#glslang#GetExecutable',
+\   'command_callback': 'ale_linters#glsl#glslang#GetCommand',
+\   'callback': 'ale_linters#glsl#glslang#Handle',
+\})