buffer: align size to stride (#50)

calculate the size to the stride we get from gb_bo_map to better align
it.

src/allocator/GBM.cpp

@@ -224,13 +224,13 @@ SDMABUFAttrs Aquamarine::CGBMBuffer::dmabuf() {
 }
 
 std::tuple<uint8_t*, uint32_t, size_t> Aquamarine::CGBMBuffer::beginDataPtr(uint32_t flags) {
-    uint32_t dst_stride = 0;
+    uint32_t stride = 0;
     if (boBuffer)
         allocator->backend->log(AQ_LOG_ERROR, "beginDataPtr is called a second time without calling endDataPtr first. Returning old mapping");
     else
-        boBuffer = gbm_bo_map(bo, 0, 0, attrs.size.x, attrs.size.y, flags, &dst_stride, &gboMapping);
-    // FIXME: assumes a 32-bit pixel format
-    return {(uint8_t*)boBuffer, attrs.format, attrs.size.x * attrs.size.y * 4};
+        boBuffer = gbm_bo_map(bo, 0, 0, attrs.size.x, attrs.size.y, flags, &stride, &gboMapping);
+
+    return {(uint8_t*)boBuffer, attrs.format, stride * attrs.size.y};
 }
 
 void Aquamarine::CGBMBuffer::endDataPtr() {