ENH: optimize the texture logic of partplate

1. reduce the icon size to 256 from 1024
2. limit the mipmap minsize to 2

Change-Id: I906f4dc2e0aa2e83e05a0d60cd0283679f41d89a

src/slic3r/GUI/GLTexture.cpp

@@ -790,7 +790,7 @@ bool GLTexture::load_from_svg(const std::string& filename, bool use_mipmaps, boo
         int lod_w = m_width;
         int lod_h = m_height;
         GLint level = 0;
-        while (lod_w > 1 || lod_h > 1) {
+        while (lod_w >= 4 && lod_h >= 4) {
             ++level;
 
             lod_w = std::max(lod_w / 2, 1);