Cubemap and HDR examples enhancements.

Fixed reflection on examples.
HDR now uses a expose / gamma tone mapping.
HDR example images.
Revert "HDR example images."

This reverts commit 787c8ab6b7c71c24ca1e34187cffe6b2b33f6a3f.
Replaced both HDR image file formats.
Code cleaning on examples.

Removed magic constants by fixing calculations.
Removed debug code.
Accept .hdr files.

But HDR example still uses .png RGBE files because of better compression ratio, transfer speed and decompression speed.
HDR examples comments.
Merge pull request #22 from zqsd/develop

HDR and Cubemap examples improvements.