delete[] was being called for a malloc() allocated C buffer.
Found by valgrind:
```
==23065== Mismatched free() / delete / delete []
==23065== at 0x4C2F77B: operator delete[](void*) (vg_replace_malloc.c:621)
==23065== by 0x50C5529: std::default_delete<unsigned char []>::operator()(unsigned char*) const (unique_ptr.h:119)
==23065== by 0x523EC85: std::unique_ptr<unsigned char [], std::default_delete<unsigned char []> >::reset(unsigned char*) (unique_ptr.h:581)
==23065== by 0x523E3D4: mbgl::Raster::upload(mbgl::gl::ObjectStore&) (raster.cpp:63)
==23065== by 0x520CA08: mbgl::RasterBucket::upload(mbgl::gl::ObjectStore&) (raster_bucket.cpp:16)
==23065== by 0x520E56A: mbgl::Painter::render(mbgl::style::Style const&, mbgl::FrameData const&, mbgl::SpriteAtlas&) (painter.cpp:130)
==23065== by 0x52BE15A: mbgl::Map::Impl::render() (map.cpp:265)
==23065== by 0x52BD9BE: mbgl::Map::render() (map.cpp:175)
==23065== by 0x50C9EB4: QMapboxGL::render() (qmapboxgl.cpp:579)
==23065== by 0x4056D3: MapWindow::paintGL() (mapwindow.cpp:166)
```
This reverts commit 6709bdcacd.
There was an issue with the script that removes '-lwebp' from WebP
linker flags, since we're statically linking. This is now fixed.
