Merge branch 'release-0.39.0' into develop
Update of web page

- Updated index page and CSS.
- Updated news feed source.
Updated the XPM image format loader

- Added support for non-standard 3D XPM images.
Multiple changes

- Moved certain declarations into separate new files, for more
  lightweight header dependencies.
- Added ifdefs for partial compatibility with GL ES 3.0.
Multiple changes

- Reimplemented the images::FilteredImage template class.
- Renamed the image::Transformed class to image::TransformComponents.
- Added the images::FlipImageAxes image filter.
- Added new constructors to Vector<T, N>.
Cleanup of images::FilteredImage
Added GetImage to Texture and DSATextureEXT

- Closes #34.
Bugfix in images::FilteredImage
Documentation updates
Multiple changes

- Moved the texture-related enumerations into separate headers.
- Moved the implementations of Texture functions into the implement/ directory.
Moved Texture functions to the library

- Moved some Texture functions to the include/ directory.
Multiple changes

- Added preliminary support for GL|ES3 header and library detection.
- Added a standalone example using GLES3/gl3.h as the GL API header.
- Minor update in texture.ipp make it compatible with GL|ES3.
Refactored Buffer::TypedMap

- The Buffer::TypedMap<T> nested template was split into two not-nested
  classes BufferRawMap and BufferTypedMap<T>.
- The examples were updated accordingly.
Moved Buffer-related things into separate files

- Moved the Buffer-related enumerations and the BufferRawMap and BufferTypedMap
  classes into separate files.
Minor fix in external library detection
Moved several classes into separate files
Moved some error-handling functions into the lib
Moved the binding-query functions to the library
Minor improvement of compatibility GLES3
Minor compatibility improvements