On some Mali devices we have encountered driver crashes caused by
calling textureSize(samplerExternalOES) in a shader without also
potentially sampling from the texture in the shader. ARM's suggested
workaround was to trick the driver in to thinking that the texture may
be sampled from (ie by sampling in a branch which is never dynamically
taken).

This is done by checking the value of a dummy uniform, and sampling
the texture if the value is non-default. Using a constant expression
did not work because the compiler would optimize the condition (and
therefore the sample) away.

Also re-enable webrender on Mali-72 and G76 devices, as it was blocked
due to this bug.

Differential Revision: https://phabricator.services.mozilla.com/D105493