Fix some error:
* glViewport not controlled by framebuffer, and need to set manually when
target framebuffer is changed.
* change even-odd stencil operation, so no need to do third draw call to
clear stencil buffer
* fix the missing `GlCanvas::update` calls in Lottoe and LottieExtension
Selecting a tool without choosing the required tools
resulted in a usage error. Now, the activation
of the appropriate saver/loader for a given tool
is enforced.
examples are not considered a feature of ThorVG;
hence, they are excluded from the src directory.
This change allows developers to concentrate more effectively
on the core ThorVG sources for practical usages.
