use glReadPixels for compatibility with OpenGL ES

src/texture.cpp

@@ -115,7 +115,29 @@ Image Texture2D::get_image() const noexcept
     App::I->render_task([&]
     {
         bind();
-        glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, ret.m_data.get());
+        
+        GLuint fboID;
+        glGenFramebuffers(1, &fboID);
+        if (fboID == 0)
+            return;
+
+        GLint oldFboID;
+        glGetIntegerv(GL_DRAW_FRAMEBUFFER_BINDING, &oldFboID);
+
+        glBindFramebuffer(GL_FRAMEBUFFER, fboID);
+        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_tex, 0);
+        GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
+        if (status == GL_FRAMEBUFFER_COMPLETE)
+        {
+            glReadPixels(0, 0, m_width, m_height, GL_RGBA, GL_UNSIGNED_BYTE, ret.m_data.get());
+        }
+        else
+        {
+            LOG("Texture2D::get_image() failed because framebuffer status = %d", (int)status);
+        }
+        
+        glBindFramebuffer(GL_FRAMEBUFFER, oldFboID);
+        glDeleteFramebuffers(1, &fboID);
     });
     return ret;
 }