GPUShader: Fix linking working even if one shader compilation failed

Linking without valid shaders works on some drivers. Avoid this case by
forcing linking step to return false.

source/blender/gpu/opengl/gl_shader.cc

@@ -151,6 +151,7 @@ GLuint GLShader::create_shader_stage(GLenum gl_stage, MutableSpan<const char *>
   }
   if (!status) {
     glDeleteShader(shader);
+    compilation_failed_ = true;
     return 0;
   }
 
@@ -193,6 +194,10 @@ void GLShader::fragment_shader_from_glsl(MutableSpan<const char *> sources)
 
 bool GLShader::finalize(void)
 {
+  if (compilation_failed_) {
+    return false;
+  }
+
   glLinkProgram(shader_program_);
 
   GLint status;

source/blender/gpu/opengl/gl_shader.hh

@@ -39,10 +39,12 @@ class GLShader : public Shader {
  private:
   /** Handle for full program (links shader stages below). */
   GLuint shader_program_ = 0;
-
+  /** Individual shader stages. */
   GLuint vert_shader_ = 0;
   GLuint geom_shader_ = 0;
   GLuint frag_shader_ = 0;
+  /** True if any shader failed to compile. */
+  bool compilation_failed_ = false;
 
   eGPUShaderTFBType transform_feedback_type_ = GPU_SHADER_TFB_NONE;