add changed needed to lower sdk to 21

[mjtalbot]

draft because it does not work (yet?)

(but this resolves other issues we would run into along the way to api v 21)

we run into this issue:

06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98] JNI ERROR (app bug): local reference table overflow (max=512)
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98] local reference table dump:
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]   Last 10 entries (of 512):
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       511: 0x71368f98 java.lang.Class<android.graphics.Paint$Style>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       510: 0x71368f98 java.lang.Class<android.graphics.Paint$Style>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       509: 0x7136bcf8 java.lang.Class<android.graphics.Paint>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       508: 0x7136bcf8 java.lang.Class<android.graphics.Paint>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       507: 0x12f1edd0 android.graphics.Paint
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       506: 0x7136bcf8 java.lang.Class<android.graphics.Paint>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       505: 0x7136bcf8 java.lang.Class<android.graphics.Paint>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       504: 0x712da4a0 android.graphics.Paint$Style
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       503: 0x71368f98 java.lang.Class<android.graphics.Paint$Style>
06-03 13:45:59.395 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       502: 0x71368f98 java.lang.Class<android.graphics.Paint$Style>
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]   Summary:
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]        54 of android.graphics.Paint (54 unique instances)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]         3 of java.lang.String (3 unique instances)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]         3 of android.graphics.Paint$Cap (2 unique instances)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]         2 of android.graphics.Path (2 unique instances)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]         1 of app.rive.runtime.kotlin.RiveAnimationView
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]         1 of java.lang.String[] (3 elements)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]         3 of android.graphics.Paint$Join (1 unique instances)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]       392 of java.lang.Class (7 unique instances)
06-03 13:45:59.410 4378-4378/app.rive.runtime.example A/art: art/runtime/indirect_reference_table.cc:98]        53 of android.graphics.Paint$Style (2 unique instances)

[mjtalbot]

pretty sure this is related to us creating lots of objects (sure) but also how we refer to native classes from within the cpp bindings...

so we used to create these references once. and remember them. but they're not thread safe, so we now grab then on the fly. we do no cleanup here at the moment

e.g.

	jclass getClass(const char *name)
	{
		return static_cast<jclass>(getJNIEnv()->FindClass(name));
	}

https://stackoverflow.com/questions/20917777/jni-error-local-reference-table-overflow-512-entries

[mjtalbot]

there are still some display issues, as not all blend modes are supported in < 29 & multiply seems to behave grossly different, but this enables compatibility!