Painter updates: scaling (HAVE_SCALING=1) and clip rect on clear

[nstbayless]

Summary

• pntr_clear is now affected by the clip rect, as per the love 2d api documentation.
• build with make HAVE_SCALING=1 to enable scaling in pntr_draw. (This allows images to be scaled when drawn)
• draw offset is applied correctly now (subtracted, not added)

[RobLoach]

This is great! Thanks for the addition... Would you mind adding a test or an example for this somewhere? Also, I thought some of this was handled by HAVE_COMPOSITION when drawing images when scaled? Perhaps that was different.

[nstbayless]

@RobLoach, I've now added a unit test for scaling (test/graphics/scale.lua). It also tests that image offset is correctly applied (the ox, oy field in love.graphics.draw). Image offset was previously applied incorrectly -- it should be subtracted, not added. There's a nice explanation whhy here.

Also, I thought some of this was handled by HAVE_COMPOSITION when drawing images when scaled?

From the looks of the code, HAVE_COMPOSITION only affects how transparency is resolved. From the README:

make HAVE_COMPOSITION=1` Enables alpha-blending.

By the way, one thing I'm not sure about is how to check from the lua test side of things if lutro was compile with HAVE_TRANSFORM (or HAVE_COMPOSITION). Ideally the scaling test should be skipped if HAVE_COMPOSITION is disabled (or the makefile should ensure that tests are built with both fields enabled somehow).

[RobLoach]

Hmmm, we could possibly expose something like love.config.transform or love.config.composition or something?

[lgtm-com[bot]]

This pull request introduces 1 alert when merging e4f931df04ee916d0446b8313bec689e2dfbd699 into 81e078f29795b89d597ab93ebf56d35fe72be99d - view on LGTM.com

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[nstbayless]

Hmmm, we could possibly expose something like love.config.transform or love.config.composition or something?

Okay, I've added this. See diff in lutro.c.

[RobLoach]

@nstbayless Apologies for being away. Is this good to merge???? Looks pretty awesome.

[skrat]

Is this going to be merged?