feat(gl_loader): añadir glActiveTexture y glGenerateMipmap

Necesarios para que gl_texture_load (cpp/functions/gfx/) funcione en
Windows tras wglGetProcAddress. En Linux son simbolos directos via
GL_GLEXT_PROTOTYPES, no afecta.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cpp/functions/gfx/gl_loader.cpp

@@ -32,6 +32,8 @@ PFNGLUNIFORM3FPROC           fn_glUniform3f           = nullptr;
 PFNGLUNIFORM4FPROC           fn_glUniform4f           = nullptr;
 PFNGLUNIFORM4FVPROC          fn_glUniform4fv          = nullptr;
 PFNGLUSEPROGRAMPROC          fn_glUseProgram          = nullptr;
+PFNGLACTIVETEXTUREPROC       fn_glActiveTexture       = nullptr;
+PFNGLGENERATEMIPMAPPROC      fn_glGenerateMipmap      = nullptr;
 
 namespace fn::gfx {
 
@@ -70,6 +72,8 @@ bool gl_loader_init() {
     LOAD(glUniform4f);
     LOAD(glUniform4fv);
     LOAD(glUseProgram);
+    LOAD(glActiveTexture);
+    LOAD(glGenerateMipmap);
 
     #undef LOAD
     return true;