backends/native: Declare variables at beginning of block
In consistence with the code style, and in order to fix build errors with older clang:
../src/backends/native/meta-onscreen-native.c:521:7: error: expected expression
521 | graphene_rect_t src_rect;
| ^
../src/backends/native/meta-onscreen-native.c:529:39: error: use of undeclared identifier 'src_rect'; did you mean 'dst_rect'?
529 | &src_rect);
| ^~~~~~~~
| dst_rect
../src/backends/native/meta-onscreen-native.c:522:20: note: 'dst_rect' declared here
522 | MtkRectangle dst_rect;
| ^
And warnings with newer clang:
../src/backends/native/meta-onscreen-native.c:521:7: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
521 | graphene_rect_t src_rect;
| ^
This should allow the build for coverity to succeed again.
Fixes: adc776d0 ("crtc/kms: Pass on src and dst rects to primary plane assignments")