cogl: make sure to init ctx->framebuffers
cogl: make sure to init ctx->framebuffers

This makes sure to initialize ctx->framebuffers to NULL otherwise we can
get apps crashing when they create their first framebuffer.

Distrotech/cogl,djdeath/cogl-android,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,spatulasnout/cogl,Distrotech/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl,spatulasnout/cogl,gcampax/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl,gcampax/cogl,collects/cogl,collects/cogl