Make sure to make the GL context is current before destroying the femtovg canvas.
The drop implementation of the canvas will call functions to delete GL resources such as textures, FBOs, etc.
and for that to work reliably the context must be current.
It can be activated using the `SIXTYFPS_DEBUG_PERFORMANCE` environment
variable, which takes a comma separated list of options.
This could be extended in the future with different locations for the
overlay (corner_top_left, corner_top_right, etc.) - but this is a simple
start.
cc #728
Separate the image cache from the texture cache.
The latter needs additional data in the key. The two have less in common that it seemed.
This also fixes a small issue with `CachedImage::upload_to_gpu` that for
SVGs would return a decoded (rendered) SVG image, instead of a texture.
That was harmless because we always call `ensure_uploaded_to_gpu`
later, but now we assert that the texture cache only holds textures,
because clear() (former remove_textures()) just clears everything.
The initial clipping was set in physical size rather than in logical size.
When scaling up, this does not have visible effect, but when scaling down
it clips too much.
