Fix rendering bugs & improve error handling.

Note: Various debugging logging statements from
350b20bdb983622bcd67b3c607e0fced8c2b63b6 are still in place until the
bug from #203 can be verified as being fixed.

Bugfixes:

- The checkTarget method in CoreTexture2D had previously been re-written
  to use if statements rather than a switch because the switch cases had
  to be converted to methods, which is illegal for a switch statement.
  Unfortunately during this re-writing a logic error was made that used
  AND's (&&) instead of OR's (||) for the conditions that used to be
  cases. This had the effect of throwing a CoreGLException during the
  creation of new textures, causing the texture id of a new texture atlas
  to be -1, which in turn caused a NullPointerException in
  BatchRenderBackendCoreProfileInternal when attempting to manipulate a
  non-existent texture atlas. To make matters worse, the exception was
  being swallowed by the LoggerShortFormat logging formatter that is used
  by the nifty-examples, preventing easy debugging. Change checkTarget
  method in CoreTexture2D to use the correct OR (||) logic, and fix the
  nifty-examples logging formatter to not swallow the exception (more on
  the logging in the "Improved error handling" section). Fixes bug from
  #203.

- The glTextImage2D method in CoreTexture2D was ignoring the
  internalFormat parameter being passed to this method, and was
  assigning the internal field this.texImageInternalFormat to the format
  parameter instead, which basically means that the internal texture
  format of newly created textures is being assigned to the format of the
  pixel data when it should be being assigned to the internalFormat
  parameter, which specifies the format of the texture. Of course, this
  bug never manifested in normal use of the CoreTexture2D class because
  the most common pixel data format, GL_RGBA, just happens to match the
  most common internal texture format, GL_RGBA. This bug would have
  manifested more clearly if compressed textures were being used, where
  the pixel data format would differ from the internal texture format.
  Assign this.texImageInternalFormat to the correct parameter,
  internalFormat.  This bug appears to have never been documented, and
  also seems to have been around for a very long time.

Improved error handling:

- Change LoggerShortFormat logging format used by nifty-examples to
  include the stack trace of any exception that is logged. The logger
  was swallowing exceptions whole o_O...

- Remove duplicate OpenGL error checking methods from
  BatchRenderBackendInternal, BatchRenderBackendCoreProfileInternal, &
  CoreTexture2D - use CheckGL class instead as it provides the exact
  methods.

- Remove duplicate texture size checking method from
  BatchRenderBackendInternal, BatchRenderBackendCoreProfileInternal, &
  CoreTexture2D - move it to CheckGL class instead so that all
  OpenGL-based internal rendering classes can make use of it. There are
  other checking-type methods in CoreTexture2D but they are only used
  internally by that class and so haven't been moved to CheckGL.

- Add OpenGL error checking to CoreProfileSaveGLState because it makes a
  lot of OpenGL calls.

- Change log warning entries in BatchRenderBackendInternal &
  BatchRenderBackendCoreProfileInternal to severe when dealing with
  failed texture creation or failed texture updating to make them stand
  out more, and this is really a more appropriate level for such
  phenomenon, and add more detailed debugging messages to these log
  entries.

Miscellaneous :

- Changed order of constructors in CoreTexture2D so they are basically
  in order of increasing complexity.

- Remove unused method from CoreTexture2D.

- Rearranged imports in LwjglGL, LwjglInitHelper, & NiftyTest to be in
  correct order.

- Remove some unnecessary this keywords.

- Add "Internal implementations" comments in CheckGL & LoggerShortFormat.