iOS OpenGL Texture 转为 UIImage 的方法

调用该方法前，需要先绑定纹理并且设置为激活状态！

- (UIImage *)imageFromTextureWithwidth:(int)width height:(int)height {
    // glActiveTexture(GL_TEXTURE1); 先绑定某个纹理
    int size = width * height * 4;
    GLubyte *buffer = malloc(size);
    glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, size, NULL);
    int bitsPerComponent = 8;
    int bitsPerPixel = 32;
    int bytesPerRow = 4 * width;
    CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
    CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
    CGImageRef imageRef = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
    UIImage *image = [UIImage imageWithCGImage:imageRef];
    free(buffer);
    return image;
}