Enhance Makefile, add pkg-config support

Improve the Makefile and add pkg-config support, so that a stand-
alone application only needs the sre pkg-config link flags,
corresponding to the version of SRE installed (which may vary
depending on SRE library configuration options and back-ends).

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Fix erratic view change at start-up and window resizing

Fix bug that caused erratic camera view changes at start-up (after
changing mousing panning mode) and when resizing the window.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>
Add function to load mipmapped texture while skipping large levels

Add the sreCreateTextureLimitLevel() function to allow the mipmap
level that is actually used to be limited to a given size at the
texture level. Only the mipmaps levels in the allowable size range
are uploaded to the GPU. Previously, detail reduction only happened
in conjunction with global texture detail flags.

To facilitate this, sreTexture::Load() has been added. The
sreCreateTexture() functions are the preferred way to create
textures, while directly invoking the constructor is deprecated
for use applications.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>