gl_engine: -- compiler warnings

gl_engine: -- compiler warnings

../src/lib/gl_engine/tvgGlRenderer.cpp: In member function ‘virtual bool GlRenderer::renderShape(tvg::RenderData)’:
../src/lib/gl_engine/tvgGlRenderer.cpp:171:30: warning: ‘a’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  171 |                 drawPrimitive(*sdata, r, g, b, a, i, RenderUpdateFlag::Stroke);
      |                 ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/lib/gl_engine/tvgGlRenderer.cpp:171:30: warning: ‘b’ may be used uninitialized in this function [-Wmaybe-uninitialized]
../src/lib/gl_engine/tvgGlRenderer.cpp:171:30: warning: ‘g’ may be used uninitialized in this function [-Wmaybe-uninitialized]
../src/lib/gl_engine/tvgGlRenderer.cpp:171:30: warning: ‘r’ may be used uninitialized in this function [-Wmaybe-uninitialized]
../src/lib/gl_engine/tvgGlRenderer.cpp: In member function ‘virtual void* GlRenderer::prepare(const tvg::RenderShape&, tvg::RenderData, const tvg::RenderTransform*, uint32_t, tvg::Array<void*>&, tvg::RenderUpdateFlag, bool)’:
../src/lib/gl_engine/tvgGlRenderer.cpp:215:21: warning: ‘alphaS’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  215 |     uint8_t alphaF, alphaS;

Change-Id: I65d6cc0b6449342232287f295c06ef640e50b389