WIP: OpenGL: Specialization Constants

This PR adds support for specialization constants for the OpenGL
backend.

**TODO**

* [ ] Don't store sources when there is no specialization constants
      in the shader interface
* [ ] Separate constants from resources as this would be the only
      part of the shader that is recreated.

@@ -123,6 +123,11 @@ class ShaderInterface {
     return input_lookup(
         inputs_ + attr_len_ + ubo_len_ + uniform_len_ + ssbo_len_, constant_len_, name);
   }
+  inline const ShaderInput *constant_get(const int binding) const

@@ -678,0 +658,4 @@
+  for (int constant_index : IndexRange(constants.types.size())) {
+    const char *name = nullptr;
+    /*
+     * When compiling GL shaders with specialization constants there is a dependency cycle.

@@ -1412,0 +1568,4 @@
+  }
+}
+
+void GLShader::SpecializationPrograms::ensure_program_linked(GLShader &shader)

@@ -21,0 +56,4 @@
+  Vector<const char *> update_constants_source(const StringRefNull constants_patch) const;
+
+ private:
+  void update_patch_index();

@@ -33,3 +75,1 @@
-  GLuint geom_shader_ = 0;
-  GLuint frag_shader_ = 0;
-  GLuint compute_shader_ = 0;
+  struct SpecializationProgram {

@@ -35,1 +75,3 @@
-  GLuint compute_shader_ = 0;
+  struct SpecializationProgram {
+    /** Handle for program. */
+    GLuint shader_program = 0;

@@ -36,0 +84,4 @@
+    void link(Shader &shader);
+  };
+
+  struct SpecializationPrograms {

@@ -36,0 +88,4 @@
+    using Key = Vector<shader::ShaderCreateInfo::SpecializationConstant::Value>;
+
+   private:
+    Map<Key, SpecializationProgram> entries;

@@ -36,0 +95,4 @@
+     * Points to the active specialization program. When binding a shader the active shader is
+     * setup.
+     */
+    SpecializationProgram *active = nullptr;

@@ -51,6 +149,7 @@ class GLShader : public Shader {
   void warm_cache(int /*limit*/) override{};
 
   std::string resources_declare(const shader::ShaderCreateInfo &info) const override;
+  std::string constants_declare(const shader::ShaderCreateInfo &info) override;

@@ -678,0 +660,4 @@
+  }
+
+  /* Add an identifier that where the specialization constants will be added. */
+  ss << "\n/* GPU_GL_SPECIALIZATION_CONSTANTS. */\n";

@@ -462,8 +467,13 @@ void GPU_shader_bind(GPUShader *gpu_shader)
     shader->bind();
     GPU_matrix_bind(gpu_shader);
     Shader::set_srgb_uniform(gpu_shader);
+    shader->constants.is_dirty = false;

@@ -324,6 +327,7 @@ GPUShader *GPU_shader_create_from_info(const GPUShaderCreateInfo *_info)
 
     Vector<const char *> sources;
     standard_defines(sources);
+    sources[SOURCES_INDEX_SPECIALIZATION_CONSTANTS] = constants.c_str();