Increased VDU throughput is crashing FabGL

[breakintoprogram]

After the latest release, it has been noted that:

• sprites_5.bas crashes after a couple of minutes
• sprites_4.bas crashes, though it will run a lot longer before it does
• sprites_3.bas is fine; that only has 1 frame per sprite
• triangles.bas crashes in MODE 2 after 30 seconds or so

This card has been forked from: breakintoprogram/agon-bbc-basic#13

[breakintoprogram]

This is looking like the increased throughput of the comms is overflowing something in FabGL.

Potential fix:

VGAController->enableBackgroundPrimitiveExecution(true)
VGAController->enableBackgroundPrimitiveExecution(false)
And also:

Canvas->waitCompletion(false) after lengthy drawing operations like triangle, circle

[breakintoprogram]

Testing:

• sprites_5.bas: Pass (60 minutes)
• sprites_4.bas: Pass (60 minutes)
• triangles.bas (in MODE 2): Pass (60 minutes)

This is after moving the Canvas->waitCompletion(false) to run once after every call to the function vdu, possibly at the expense of a bit more flicker. Note that calling that function with the parameter true causes the VPD to run really slow.