bgfx: fix shader compilation

[bkmeneguello]

Fixes #5105

• Adds ".sc" as a supported extension for bgfx shaders;
• Adds support for vs_, fs, and cs_ prefixes for vertex, fragment, and compute shaders;
• Adds support for bin2c confguration (header generation)

[waruqi]

use package:is_plat

[xq114]

I think there should be more info about .header argument in the comments at the beginning of this file, e.g. What would happen if both -o <output> and --bin2c <header> are specified? Which of the <output> and the <header> will be generated? Is output_name = "shader.vert.h" really correct?

[bkmeneguello]

The --bin2c option generates a header file containing the compiled shader to be included in the project instead of loaded in runtime. It the output_name is defined, the ".h" suffix is recommended, but it just generates a header file. The "

" string is optional and, if provided it's used as the variable containing the shader data, if not defined it uses the input file base name. Since not all rule configuration values follow strictly the shaderc inputs I didn't follow also and used a more explicit name. What is the best way to document this rule, if it's necessary to read the source anyway, is that worth documenting it better?

[xq114]

Of course it's better documenting the rule on some website like xrepo.xmake.io, but it's not yet implemented, so as for now the only way to document it is to do so in the rule file. Seems that when there is --bin2c <header>, the variable name in the generated file will be <header>? If so, I'd suggest a different name for this config, e.g. export_var or array_name(align with https://bkaradzic.github.io/bgfx/tools.html#shader-compiler-shaderc ) or just array to be more clear.

[xq114]

array_name=true can become an advanced feature (deduce by extension .h is usually enough), so it's ok to document in the following way:

-- add_rules("@bgfx/shaders")
-- generate binary file:
-- add_files("shader.vert", {type = "vertex", output_dir = "shaders", output_name = "shader.vert.bin", profiles = {glsl = "330"}})
-- generate header file:
-- add_files("vs_shader.sc", {type = "vertex", output_dir = "shaders", output_name = "shader.vert.h", profiles = {glsl = "330"}})
-- generate header file exporting variable "vertex_src":
-- add_files("vs_shader.sc", {type = "vertex", output_dir = "shaders", output_name = "shader.vert.h", array_name = "vertex_src", profiles = {glsl = "330"}})
-- force to generate header file with default variable name:
-- add_files("vs_shader.sc", {type = "vertex", output_dir = "shaders", output_name = "shader.vert.inc", array_name = true, profiles = {glsl = "330"}})