- split out the span generation from most texture classes

Until now each subclass of FTexture had to implement the entire span generation itself, presumably so that a few classes can use simpler structures.
This does not work if a texture can have more than one pixel buffer as is needed for alpha textures.
Even though it means that some classes will allocate more data now, it's the only way to do it properly.
In addition this removes a significant amount of mostly redundant code from the texture classes.

- added alpha texture processing to all converted classes

As of now this is not active and not tested.
Note that as part of the conversion even those textures that were working as alphatextures will not look correct until the higher level code gets adjusted.

src/gl/textures/gl_texture.cpp

@@ -523,24 +523,11 @@ FBrightmapTexture::FBrightmapTexture (FTexture *source)
 	SourceLump = -1;
 }
 
-FBrightmapTexture::~FBrightmapTexture ()
-{
-}
-
-const uint8_t *FBrightmapTexture::GetColumn (unsigned int column, const Span **spans_out)
-{
-	// not needed
-	return NULL;
-}
-
-const uint8_t *FBrightmapTexture::GetPixels ()
-{
-	// not needed
-	return NULL;
-}
-
-void FBrightmapTexture::Unload ()
+uint8_t *FBrightmapTexture::MakeTexture(FRenderStyle style)
 {
+	// This function is only necessary to satisfy the parent class's interface.
+	// This will never be called.
+	return nullptr;
 }
 
 int FBrightmapTexture::CopyTrueColorPixels(FBitmap *bmp, int x, int y, int rotate, FCopyInfo *inf)