Enable DTX1 and 3 on RW GL (where applicable)

DTX1 and DTX3 is supported by Linux drivers for long time and
it can be enable at least where is the hardware support available.

- improves first time loading
- save about 183 MB on storage
- saves GPU memory (not measured)
- brings slightly performance boost (1-2 fps on Intel HD 620)

src/rw/TexRead.cpp

@@ -294,7 +294,7 @@ CanVideoCardDoDXT(void)
 #ifdef LIBRW
 	// TODO
 #ifdef RW_OPENGL
-	return false;
+	return rw::gl3::gl3Caps.dxtSupported;
 #else
 	return true;
 #endif