Regression from PR #3795.

DriveReader::m_size was never initialized which was indirectly
causing CGameListCtrl to crash Dolphin when it tried to insert a
character at a negative index in a string.

Reading one sector at a time is very inefficient and appears to
be causing timing issues during boot so SectorReader has been
enhanced to support batching.

SectorReader has been given a working cache system.

ES_OPENCONTENT and ES_CLOSECONTENT now call Open and Close respectively,
as the old code did.

instead, leave all the management with the NANDContentLoader.

for file data (directly on the NAND), this opens the file on-demand and
returns the requested chunk when asked for it.
for on-the-fly decrypted WAD data, we just keep the decoded buffer in
memory, like we've done before - except that we don't give away any objects
we don't want to.

Debug Builds: Fixed vector assertion

Using the XPM format for images has become a maintenance problem because
people don't know how to create them. This commit removes all XPM images
and all C files that contain PNG images. DolphinWX now uses the PNGs
in the Resources folder instead, just like DolphinQt and DolphinQt2 do.

NANDContentLoader/WiiWAD: Get rid of raw delete and new