Change the default pixel format from RGB to RGBA for render texture when saving as PNG.
Merge pull request #7049 from dabingnn/v3_fixDeleteTexture

fix bug for deleteTexture
Make the API more extendable
The filename should be converted to lower case before comparing.
[ci skip]
Merge pull request #7056 from visiblelight/render_texture

Use RGBA when saving a file from render texture